Wonder at his men’s not unusual (12)
I believe the answer is:
astonishment
'wonder' is the definition.
(I've seen this before)
'at his men's not unusual' is the wordplay.
'unusual' indicates an anagram.
'at'+'his'+'mens'+'not'='athismensnot'
'athismensnot' with letters rearranged gives 'ASTONISHMENT'.
(Other definitions for astonishment that I've seen before include "Stupefaction" , "Amazement" , "confusion" , "Wonder" , "Great surprise" .)