Slip into a slightly lisping affectation (5)
I believe the answer is:
gaffe
'slip' is the definition.
(both can mean an error or blunder)
'into a slightly lisping affectation' is the wordplay.
'into' indicates the answer is hidden within the clue.
'GAFFE' is hidden in the letters of 'slightly lisping affectation'.
(Other definitions for gaffe that I've seen before include "Remark one wishes unmade" , "A foolish error" , "Brick (that's dropped)" , "Blundering remark" , "Indiscretion" .)