Old Bob made a meal of fish (4)
I believe the answer is:
shad
'fish' is the definition.
(I know that shad is a type of fish)
'old bob made a meal of' is the wordplay.
'old bob' becomes 's' (abbreviation for bob or shilling used before 1971).
'made a meal of' becomes 'had' (I've seen this in another clue).
's'+'had'='SHAD'
(Other definitions for shad that I've seen before include "Herring-like fish" , "Fish; dash (anag.)" , "Fish related to the herring" , "fish that's been caught" .)