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' is a charade indicator (letters next to each other).
'the girl' becomes 'Vi' (short for Violet).
'to agree' becomes 'tally' (synonyms).
'vi' next to 'tally' is 'VITALLY'.
(Other definitions for vitally that I've seen before include "Essentially" , "crucially" .)