But catching one is no credit to a sailor! (4)
I believe the answer is:
crab
'but catching one is no' is the definition.
I can't tell whether this definition defines the answer.
'credit to a sailor' is the wordplay.
'credit' becomes 'cr'.
'to a sailor' becomes 'ab' (abbreviation for able seaman).
'cr'+'ab'='CRAB'
Can you help me to learn more?
(Other definitions for crab that I've seen before include "Seafood (that can be caught by a rower?)" , "Clawed shellfish" , "A crustacean; type of apple" , "Food from the sea" , "Ten-legged shellfish" .)