Speaker's dainty half-sister (7)
I believe the answer is:
tweeter
'speaker's' is the definition.
(tweeter is a kind of speaker)
'dainty half-sister' is the wordplay.
'dainty' becomes 'twee' ('twee' can be a synonym of 'dainty').
'half' means to take half the letters.
'sister' halved is 'ter'.
'twee'+'ter'='TWEETER'
(Other definitions for tweeter that I've seen before include "Robin, for one" , "Message sender" , "one delivering a short message?" , "hi-fi unit?" , "Poster of short message - little bird?" .)