Just beat one shellfish (4)
I believe the answer is:
pipi
'shellfish' is the definition.
(I have seen 'edible shellfish' mean 'pipi' so perhaps 'shellfish' could also mean 'pipi')
'just beat one' is the wordplay.
'just beat' becomes 'pip' (as in pipping someone to the post).
'one' becomes 'i' (Roman numeral).
'pip'+'i'='PIPI'
(Other definitions for pipi that I've seen before include "pods" , "Brazilian pod" , "edible shellfish" .)