Set against warrant officer, everyone appears insignificant? (5)
I believe the answer is:
small
'appears insignificant?' is the definition.
(something small appears insignificant)
'set against warrant officer everyone' is the wordplay.
'set against' says to put letters next to each other.
'warrant officer' becomes 'sm' (Sergeant Major).
'everyone' becomes 'all' (I've seen this before).
'sm' put next to 'all' is 'SMALL'.
(Other definitions for small that I've seen before include "Of little size" , "Piddling" , "Unimportant; young" , "Diminutive" , "Petite" .)