Marine food concession may welcome visit (7)
I believe the answer is:
scallop
'marine food' is the definition.
'concession may welcome visit' is the wordplay.
'concession' becomes 'sop' (sop is a kind of concession).
'may welcome' means one lot of letters goes inside another.
'visit' becomes 'call' ('call' can be a synonym of 'visit').
'sop' enclosing 'call' is 'SCALLOP'.
(Other definitions for scallop that I've seen before include "Shell - sign of a pilgrim" , "Bivalve with radiating ridges on shell" , "Edible bivalve mollusc" , "Marine bivalve" , "Shapely dish" .)