Fat woman on top of tram (4)
I believe the answer is:
suet
'fat' is the definition.
(suet is a kind of fat)
'woman on top of tram' is the wordplay.
'woman' becomes 'sue' (**).
'on top of' suggests taking the first letters (I've seen 'on top' mean this).
The initial letter of 'tram' is 't'.
'sue'+'t'='SUET'
(Other definitions for suet that I've seen before include "Hard fat in cattle and sheep" , "Fatty stuff used in cooking" , "Hard white fat on sheep's kidneys" , "Hard white cooking fat" , "Hard fat from beef and sheep" .)