Milliner comes in to celebrate? Smashing! (10)
I believe the answer is:
shattering
'smashing' is the definition.
('shattering' can be a synonym of 'smashing')
'milliner comes in to celebrate?' is the wordplay.
'milliner' becomes 'hatter' ('hatter' can be a synonym of 'milliner').
'comes in to' means one lot of letters goes inside another.
'celebrate?' becomes 'sing' (I've seen this before).
'hatter' going into 'sing' is 'SHATTERING'.
(Other definitions for shattering that I've seen before include "Very shocking" , "being destructive" , "Totally exhausting; breaking" , "very upsetting" , "with devastating result" .)