Scotsman returns carrying large mollusc (4)
I believe the answer is:
clam
'mollusc' is the definition.
(clam is a kind of mollusc)
'scotsman returns carrying large' is the wordplay.
'scotsman' becomes 'mac'.
'returns' says the letters should be written backwards.
'carrying' indicates putting letters inside.
'large' becomes 'l'.
'mac' backwards is 'cam'.
'cam' going around 'l' is 'CLAM'.
(Other definitions for clam that I've seen before include "A silent type" , "Shellfish - secretive type" , "Shellfish - secretive person" , "Mollusc, one proverbially reticent" , "Mollusc with firmly-closed shell" .)