Check trousers first for Superman, underwear going on top (4)
I believe the answer is:
vest
'top' is the definition.
(I know that vest is a type of top)
'check trousers first for superman' is the wordplay.
'check' becomes 'vet' (to vet someone is to check their background).
'trousers' is an insertion indicator (trouser can mean to steal or take).
'first for' says to take the initial letters.
The first letter of 'superman' is 's'.
'vet' going around 's' is 'VEST'.
'underwear going on' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for vest that I've seen before include "American version of waistcoat" , "An American man's waistcoat" , "Place undergarment in fixed right of possession" , "Undergarment for upper body" , "Top" .)