Terrible nit the girl is - really! (2,5)
I believe the answer is:
in truth
'really' is the definition.
(I know that really can be written as in truth)
'terrible nit the girl' is the wordplay.
'terrible' indicates anagramming the letters.
'the girl' becomes 'ruth' (girl's name).
'nit' is an anagram of 'int'.
'int'+'ruth'='IN TRUTH'
'is' is the link.
(Other definitions for in truth that I've seen before include "As a matter of fact" , "Honestly" , "To be honest" , "Really" , "Actually" .)