Not trendy? Trendy? Just tell me! (3,4,2)
I believe the answer is:
out with it
'just tell me' is the definition.
('out with it' encourages someone to say something)
'not trendy? trendy?' is the wordplay.
'not trendy?' becomes 'out' (as opposed to something 'in' fashion).
'trendy?' becomes 'withit' (I've seen this before).
'out'+'withit'='OUT WITH IT!'
(Other definitions for out with it that I've seen before include "Say what's on your mind, don't suppress it" , "Stop hesitating!" , "divulge information immediately!" , "Go on" , "Tell me quickly" .)