Definitely wearing a suit (2,6)
I believe the answer is:
in spades
'definitely' is the definition.
(I know that definitely can be written as in spades)
'wearing a suit' is the wordplay.
'wearing' becomes 'in' (in jeans or in a dress).
'a suit' becomes 'spades' (playing card suit).
'in'+'spades'='IN SPADES'
(Other definitions for in spades that I've seen before include "Without restraint" , "Seriously" , "Extremely" , "To a very great degree" , "Unquestionably - how a bridge hand might be played?" .)