Essentially, get the girl to agree (7)
I believe the answer is:
vitally
'essentially' is the definition.
(I've seen this before)
'get the girl to agree' is the wordplay.
'get' says to put letters next to each other.
'the girl' becomes 'vi' (short for Violet).
'to agree' becomes 'tally' ('tally' can be a synonym of 'agree').
'vi'+'tally' is 'VITALLY'.
(Other definitions for vitally that I've seen before include "Essentially" , "crucially" .)