Mollusc caught on line in the morning (4)
I believe the answer is:
clam
'mollusc' is the definition.
(clam is a kind of mollusc)
'caught on line in the morning' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'line' becomes 'l' (used when specifying particular lines from a poem).
'in the morning' becomes 'am' (short for ante meridiem - before noon).
'c'+'l'+'am'='CLAM'
(Other definitions for clam that I've seen before include "Edible shellfish" , "Burrowing marine mollusc living in sand or mud" , "ringing noise produced by > very reticent type" , "A silent type" , "Mollusc with tightly-closed shell" .)