On reflection, be wrong (4)
I believe the answer is:
evil
'wrong' is the definition.
(I've seen this before)
'on reflection be' is the wordplay.
'on reflection' is a reversal indicator.
'be' becomes 'live' (living is a kind of being).
'live' back-to-front is 'EVIL'.
(Other definitions for evil that I've seen before include "Depraved" , "Wicked - vice" , "Depravity, wickedness" , "Badness, depravity" , "Demonic" .)