Name fashionable English square (4)
I believe the answer is:
nine
'square' is the definition.
(nine is a square number)
'name fashionable english' is the wordplay.
'name' becomes 'n' (common abbreviation e.g. on forms).
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'english' becomes 'E' (abbreviation).
'n'+'in'+'e'='NINE'
(Other definitions for nine that I've seen before include "Number IX" , "Days Lady Jane 5 ac reigned" , "Short time for a wonder" , "Number of feline existences" , "A number of worthies" .)