A nice gal’s unconventional painkiller (9)
I believe the answer is:
analgesic
'painkiller' is the definition.
(synonyms)
'a nice gal's unconventional' is the wordplay.
'unconventional' is an anagram indicator.
'nice'+'gals'='nicegals'
'nicegals' is an anagram of 'nalgesic'.
'a'+'nalgesic'='ANALGESIC'
(Other definitions for analgesic that I've seen before include "Pain reliever" , "number" , "Pain-releaving drug" , "A medicine used to relieve pain" , "Pain-killer" .)