Be leader in fashion with Italian suit (5)
I believe the answer is:
befit
'suit' is the definition.
('befit' can be a synonym of 'suit')
'be leader in fashion with italian' is the wordplay.
'leader in' says to take the initial letters.
'with' says to put letters next to each other.
'italian' becomes 'it'.
The initial letter of 'fashion' is 'f'.
'be'+'f'+'it'='BEFIT'
(Other definitions for befit that I've seen before include "Be appropriate for" , "Become, suit" , "Be right and suitable for" .)