Chap takes traitor to be inconsistent (7)
I believe the answer is:
erratic
'inconsistent' is the definition.
(I've seen this before)
'chap takes traitor' is the wordplay.
'chap' becomes 'eric' ().
'takes' is an insertion indicator.
'traitor' becomes 'rat' (rat can mean a treacherous person).
'eric' placed around 'rat' is 'ERRATIC'.
'to be' is the link.
(Other definitions for erratic that I've seen before include "Deep affection" , "Unstable" , "Irregular and unpredictable" , "Eccentric, aberrant" , "Mercurial" .)