Secretly, trendy needs self-assurance (2,10)
I believe the answer is:
in confidence
'secretly' is the definition.
(I've seen this before)
'trendy needs self-assurance' is the wordplay.
'trendy' becomes 'in'.
'needs' says to put letters next to each other.
'self-assurance' becomes 'confidence' ('confidence' can be a synonym of 'self-assurance').
'in'+'confidence'='IN CONFIDENCE'
(Other definitions for in confidence that I've seen before include "Secretly" , "privately" .)