Imperfection in headless lizard (4)
I believe the answer is:
kink
'imperfection' is the definition.
(I've seen this in another clue)
'headless lizard' is the wordplay.
'headless' means to remove the first letter.
'lizard' becomes 'skink' (skink is a kind of lizard).
'skink' with its first letter taken away is 'KINK'.
'in' is the link.
(Other definitions for kink that I've seen before include "Hitch" , "Idiosyncrasy - sharp bend" , "Sharp twist in something otherwise straight" , "Unusual sexual preference" , "Twisted loop in string" .)