Fat youth going round central Hammersmith (4)
I believe the answer is:
lard
'fat' is the definition.
(I know that lard is a type of edible fat)
'youth going round central hammersmith' is the wordplay.
'youth' becomes 'lad' (lad means a young man).
'going round' indicates putting letters inside (some letters go around others).
'central' means to look at the middle letters.
The middle letter of 'hammersmith' is 'r'.
'lad' placed around 'r' is 'LARD'.
(Other definitions for lard that I've seen before include "Clarified pig fat" , "Animal fat" , "Fat Scottish landowner appears disheartened" , "Pork fat" , "Pig's fat" .)