Fashionable suit, to a great extent (2,6)
I believe the answer is:
in spades
'to a great extent' is the definition.
(I've seen this before)
'fashionable suit' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'suit' becomes 'spades' (playing card suit).
'in'+'spades'='IN SPADES'
(Other definitions for in spades that I've seen before include "how a contract may be agreed" , "Extremely" , "Woman's garment" , "Decidedly" , "to a large degree" .)