Excess flesh (3)
'excess flesh' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for flab that I've seen before include "Soft loose flesh" , "Sign of over-indulgence" , "Ugly, surplus flesh" , "Excess body fat" , "Informally, unsightly fat" .)
(Other definitions for fat that I've seen before include "Excess body weight" , "Portly" , "Profitable" , "Substantial" , "Cooking grease" .)