Fat chap fed last of caviar (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fat)
'chap fed last of caviar' is the wordplay.
'chap' becomes 'lad' ().
'fed' means one lot of letters goes inside another (one word is 'fed' to another).
'last of' suggests the final letters.
The last letter of 'caviar' is 'r'.
'lad' placed around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "Pig fat used in cooking" , "Aggrandise - embellish" , "Clarified pig fat" , "F [FAT]" , "Pig's fat" .)