Fatty is to sit next to a girl (7)
I believe the answer is:
adipose
'fatty' is the definition.
(synonyms)
'sit next to a girl' is the wordplay.
'sit' becomes 'pose' (as in sitting for a painter).
'next to' is a charade indicator (letters next to each other).
'a girl' becomes 'adi' ().
'pose' put after 'adi' is 'ADIPOSE'.
'is to' acts as a link.
(Other definitions for adipose that I've seen before include "Is a dope fat?" , "Relating to fatty tissue" , "Relating to fat" , "Of tissue, fatty in substance" , "Of animal, fat, fatty" .)