Mollusc caught once mostly by the end of beach (5)
I believe the answer is:
conch
'mollusc' is the definition.
(conch is a kind of mollusc)
'caught once mostly by the end of beach' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'mostly' means to remove the last letter.
'by' says to put letters next to each other.
'the end of' suggests the final letters.
The final letter of 'beach' is 'h'.
'once' with its last letter taken away is 'onc'.
'c'+'onc'+'h'='CONCH'
(Other definitions for conch that I've seen before include "Type of shell" , "Big spiral-shelled mollusc" , "One's sometimes pulled in to beach" , "Big spiral shell" , "Large spiral shell" .)