Set store by mother becoming expert (7)
I believe the answer is:
maestro
'expert' is the definition.
(I've seen this before)
'set store by mother' is the wordplay.
'set' is an anagram indicator (instruction to set the letters).
'by' is a charade indicator (letters next to each other).
'mother' becomes 'ma' (ma is a kind of mother).
'store' is an anagram of 'estro'.
'estro' put after 'ma' is 'MAESTRO'.
'becoming' acts as a link.
(Other definitions for maestro that I've seen before include "Artist of consummate skill: conductor" , "Top conductor" , "Expert musician" , "Orchestral conductor" , "Rattle, say" .)