A girl's not so unpitying (8)
I believe the answer is:
ruthless
'unpitying' is the definition.
(I know that unpitying can be written as ruthless)
'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' acts as a 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" .)