Everybody on board tug is fat (6)
I believe the answer is:
tallow
'is fat' is the definition.
(I've seen this before)
'everybody on board tug' is the wordplay.
'everybody' becomes 'all' (I've seen this before).
'on board' indicates putting letters inside.
'tug' becomes 'tow' (I've seen this before).
'all' going within 'tow' is 'TALLOW'.
(Other definitions for tallow that I've seen before include "Contributor to soap" , "Animal fat used to make ..." , "Rendered fat used for making candles" , "Candle, soap, fat" , "Candle ingredient" .)