Some of the clients call openly for shellfish (7)
I believe the answer is:
scallop
'for shellfish' is the definition.
(scallop is a kind of shellfish)
'some of the clients call openly' is the wordplay.
'some of' indicates a hidden word.
'SCALLOP' is hidden in the letters of 'clients call openly'.
(Other definitions for scallop that I've seen before include "Bivalve mollusc with fan-shaped shell" , "Shell - sign of a pilgrim" , "Shapely dish" , "Marine mollusc" , "Shellfish that's also plc" .)