Clark was on the fringes of the England team this time last year when he was hit with a 32-week ban for breaking the elbow of Leicester hooker Rob Hawkins in the LV= Cup final. The suspension ran from March until November last year and ruled Clark out of England's summer tour and the autumn internationals.
Lancaster has monitored Clark's discipline closely since his return to action for Northampton and on Wednesday recalled him to the England squad for the RBS 6 Nations. He said: "I have coached him since he was 14 and I know him well enough to know that he is hugely regretful of the incident."
Lancaster added: "When he went into the hearing he didn't do anything other than put his hands up and say, 'I made a bad mistake, I apologise'. He has never felt anything other than remorse. He has served his punishment, a 32-week ban.
"I was disappointed we ended up in the situation that (he) missed out on developing himself for club and country. Massively. But equally, as with other players in the past, I think everyone deserves a second chance - and certainly he does."
Clark has replaced lock Mouritz Botha and will offer Lancaster options as both a combative back row and in the second row.
"In addition to his desire to win, his versatility in the second row is an option for us. He is incredibly athletic and very good in the contact areas," Lancaster said. "He's got to have the desire to play in the second row for him to be successful. I've checked that is the case and he's happy."
Lancaster backed Clark to make a similar impact during the Six Nations that England saw from the likes of Joe Launchbury, Tom Youngs and Mako Vunipola in the autumn. "What he's got to do is make the step from a good player of potential to an international player," Lancaster said.
Launchbury, Youngs, Vunipola and Freddie Burns have all been officially promoted into the senior squad, having been called up for the autumn Tests as injury cover. Burns has been joined by his club-mate Billy Twelvetrees, who will compete with Brad Barritt for the inside centre position, while Saracens wing David Strettle has replaced Charlie Sharples.
James Haskell returns to England's senior squad in place of Phil Dowson, who drops to the Saxons squad along with Sharples, Botha, Tom Palmer, Anthony Allen and Jordan Turner-Hall.
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