Girl belittled is without mercy (8)
I believe the answer is:
ruthless
'without mercy' is the definition.
('mercy' can be 'ruth' (as opposed to ruthlessness) so maybe 'without mercy' can be ''ruthless')
'girl belittled' is the wordplay.
'girl' becomes 'ruth' (girl's name).
'belittled' becomes 'less' (I can't justify this - if you can you should believe this answer much more).
'ruth'+'less'='RUTHLESS'
'is' is the link.
Can you help me to learn more?
(Other definitions for ruthless that I've seen before include "nasty" , "Unsparing" , "Having no mercy like hustlers" , "Harsh" , "Unmerciful" .)