Girl's a massive pain (7)
I believe the answer is:
abigail
'girl's' is the definition.
'a massive pain' is the wordplay.
'massive' becomes 'big' ('big' can be similar in meaning to 'massive').
'pain' becomes 'ail' ('ail' can be a synonym of 'pain').
'a'+'big'+'ail'='ABIGAIL'
(Other definitions for abigail that I've seen before include "domestic" , "She served David" , "Lady's maid (once); David's wife (I. Sam.)" , "Woman's name - old lady's maid" , "lady!" .)