Mistake to engage feeble reactionary (8)
I believe the answer is:
blimpish
'reactionary' is the definition.
(I've seen this before)
'mistake to engage feeble' is the wordplay.
'mistake' becomes 'bish' (informal term for an error).
'to engage' means one lot of letters goes inside another (some letters get engaged in others).
'feeble' becomes 'limp' (I've seen this before).
'bish' going around 'limp' is 'BLIMPISH'.
(Other definitions for blimpish that I've seen before include "reactionary" , "(0f a elderly person) with incurably pompous and conservative views" .)