A girl's not so unpitying (8)
I believe the answer is:
'unpitying' is the definition.
'a girl's not' is the wordplay.
'a girl' becomes 'ruth' (girl's name).
'not' becomes 'less' (I've seen this in other clues).
'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" .)