Have a meal after senior catches a fish (7)
I believe the answer is:
sardine
'fish' is the definition.
(sardine is a type of fish)
'have a meal after senior catches a' is the wordplay.
'have a meal' becomes 'dine' (I've seen this before).
'after' means one lot of letters go next to another.
'senior' becomes 'sr' (abbreviation).
'catches' is an insertion indicator.
'sr' placed around 'a' is 'sar'.
'dine' put after 'sar' is 'SARDINE'.
(Other definitions for sardine that I've seen before include "Fish in tin?" , "FRENCH FISH" , "(Cramped?) fish" , "A diner's small fish" , "Stonefish OR? Stone" .)