Show resentment when new wife is about fifty (6)
I believe the answer is:
bridle
'show resentment' is the definition.
(I've seen this in another clue)
'new wife is about fifty' is the wordplay.
'new wife' becomes 'bride'.
'is about' is an insertion indicator.
'fifty' becomes 'l' (Roman numeral).
'bride' going around 'l' is 'BRIDLE'.
'when' is the link.
(Other definitions for bridle that I've seen before include "Rein" , "Headgear used to control a horse" , "Take offence" , "Check" , "Part of horse's harness that goes on its head" .)