What supposedly makes a man check out damaged store unit (12)
I believe the answer is:
testosterone
'what supposedly makes a man' is the definition.
(male sex hormone)
'check out damaged store unit' is the wordplay.
'check out' becomes 'test' (I've seen this in another clue).
'damaged' is an anagram indicator.
'unit' becomes 'one' (synonyms).
'store' is an anagram of 'oster'.
'test'+'oster'+'one'='TESTOSTERONE'
(Other definitions for testosterone that I've seen before include "stimulator of particular sexuality" , "Male sex hormone" , "It might make a man of you" , "male driver?" .)