Fish: fifty caught by father (8)
I believe the answer is:
flounder
'fish' is the definition.
(flounder is a kind of fish)
'fifty caught by father' is the wordplay.
'fifty' becomes 'l' (Roman numeral).
'caught by' is an insertion indicator.
'father' becomes 'founder' (synonyms).
'l' put inside 'founder' is 'FLOUNDER'.
(Other definitions for flounder that I've seen before include "Have difficulties with the flatfish" , "thrash around wildly" , "Struggle clumsily - flatfish" , "Fish -- stumble helplessly" , "Small flatfish" .)