Old-maidish, but with soft edge (4)
I believe the answer is:
prim
'old-maidish' is the definition.
(thesaurus)
'with soft edge' is the wordplay.
'with' means one lot of letters go next to another.
'soft' becomes 'p' (musical abbreviation for 'piano'**).
'edge' becomes 'rim' (rim is a kind of edge).
'p'+'rim' is 'PRIM'.
'but' acts as a link.
(Other definitions for prim that I've seen before include "Prudishly disapproving" , "Stiffly correct" , "Demure" , "Proper or prissy" , "Prudish, formal" .)