A girl's not so unpitying (8)
I believe the answer is:
ruthless
'unpitying' is the definition.
(synonyms)
'a girl's not' is the wordplay.
'a girl' becomes 'ruth' (girl's name).
'not' becomes 'less' (I've seen this in other clues).
'ruth'+'less'='RUTHLESS'
'so' is the link.
(Other definitions for ruthless that I've seen before include "Having no mercy like hustlers" , "Callous" , "devoid of compassion" , "Unsparing" , "Lacking pity or mercy" .)