Smooth, yet unfashionable (4,3)
I believe the answer is:
even out
'smooth' is the definition.
(I've seen this before)
'yet unfashionable' is the wordplay.
'yet' becomes 'even' (synonyms).
'unfashionable' becomes 'out' (similar in meaning).
'even'+'out'='EVEN OUT'
(Other definitions for even out that I've seen before include "Get level" , "Become equal" , "Make less varied" , "Spread equally" , "Make flat" .)