Note about trendy dress (4)
I believe the answer is:
mini
'dress' is the definition.
(mini dress)
'note about trendy' is the wordplay.
'note' becomes 'mi' (as in do-re-mi).
'about' indicates putting letters inside.
'trendy' becomes 'in' ('in' can mean fashionable).
'mi' enclosing 'in' is 'MINI'.
(Other definitions for mini that I've seen before include "Car; short dress" , "Car designed by Issigonis" , "(Car etc) of small size" , "Little - car" , "Brief" .)