Fat boy Romeo stops (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fat)
'boy romeo stops' is the wordplay.
'boy' becomes 'lad' (informal term for boy).
'romeo' becomes 'R' (phonetic alphabet: alpha, bravo, charlie etc.).
'stops' means one lot of letters goes inside another (to stop can mean to block a gap).
'lad' going around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "Hard cooking fat" , "Pig fat used in cooking" , "Animal fat" , "Soft white semisolid fat" , "Clarified pig fat" .)