Fat girl stuck in a position ... (7)
I believe the answer is:
adipose
'fat' is the definition.
('adipose' can be similar in meaning to 'fat')
'girl stuck in a position' is the wordplay.
'girl' becomes 'di'.
'stuck in' is an insertion indicator.
'position' becomes 'pose' (posing is a kind of positioning**).
'a'+'pose'='apose'
'di' inserted into 'apose' is 'ADIPOSE'.
(Other definitions for adipose that I've seen before include "Of tissue, fatty in substance" , "Of tissue storing fat" , "Is a dope fat?" , "Composed of animal fat" , "Containing fat" .)