With team short, reserve comes on and equalises (7)
I believe the answer is:
squares
'equalises' is the definition.
(to square is to make equal)
'with team short reserve comes on' is the wordplay.
I cannot quite see how this works, but
'reserve' could be 'res' (I've seen this in other clues) and 'res' is present in the answer.
This may be the basis of clue (or it may be nonsense).
'and' is the link.
(Other definitions for squares that I've seen before include "Letters go here" , "They contain four right angles" , "Four-sided regular polygons" , "1,4,9, ..." , "Old-fashioned types" .)