Fat — pull the whole lot in (6)
I believe the answer is:
tallow
'fat' is the definition.
(thesaurus)
'pull the whole lot in' is the wordplay.
'pull' becomes 'tow' (tow is a kind of pull).
'the whole lot' becomes 'all'.
'in' means one lot of letters goes inside another.
'tow' placed around 'all' is 'TALLOW'.
(Other definitions for tallow that I've seen before include "Animal fat used to make ..." , "Melted, clarified animal fat" , "Fat used for soap/candles" , "burning fat" , "Candle grease" .)