Mother's lip on first half of bass instrument (7)
I believe the answer is:
marimba
'instrument' is the definition.
(marimba is a kind of instrument)
'mother's lip on first half of bass' is the wordplay.
'mother' becomes 'ma' (Ma is another word for mother or mum).
'lip' becomes 'rim' (synonyms).
'on' means one lot of letters go next to another.
'first half of' suggests halving the letters.
'bass' halved is 'ba'.
'ma'+'rim'+'ba'='MARIMBA'
(Other definitions for marimba that I've seen before include "Xylophone-like instr." , "Jazz instrument" , "Xylophone-like instrument from Africa" , "African xylophone" , "Latin American instrument like a xylophone" .)