Fat model gets permit (6)
I believe the answer is:
tallow
'fat' is the definition.
(fat used to make candles)
'model gets permit' is the wordplay.
'model' becomes 't' (Ford Model T car).
'gets' means one lot of letters go next to another.
'permit' becomes 'allow' (synonyms).
't'+'allow'='TALLOW'
(Other definitions for tallow that I've seen before include "Fatty substance used in candles, soap etc" , "Animal fat used to make ..." , "Rendered fat used for making candles" , "Melted, clarified animal fat" , "burning fat" .)