Fail to counterfeit a letter (6)
I believe the answer is:
forget
'fail' is the definition.
('forget' can be a synonym of 'fail')
'counterfeit a letter' is the wordplay.
'counterfeit' becomes 'forge' ('forge' can be a synonym of 'counterfeit').
'a letter' becomes 't' (**).
'forge'+'t'='FORGET'
'to' is the link.
(Other definitions for forget that I've seen before include "Slip one's mind" , "Don't remember" , "Stop remembering" , "Stop thinking about" , "Lose memory of" .)