On us, perhaps, it will fit (4)
I believe the answer is:
suit
'fit' is the definition.
(I know that fit can be written as suit)
'on us perhaps it' is the wordplay.
'on' says to put letters next to each other.
'perhaps' is an anagram indicator.
'us' anagrammed gives 'su'.
'su'+'it' is 'SUIT'.
'will' acts as a link.
(Other definitions for suit that I've seen before include "Legal action - courtship" , "Formal attire" , "Become" , "fit" , "Eg, hearts or clubs" .)