Surveys returned about calcium in seafood (7)
I believe the answer is:
scallop
'seafood' is the definition.
(scallop is a kind of seafood)
'surveys returned about calcium' is the wordplay.
'surveys' becomes 'polls' (polling is a kind of surveying).
'returned' shows that the letters should be reversed in order.
'about' means one lot of letters goes inside another.
'calcium' becomes 'ca' (Ca is the chemical symbol for calcium).
'polls' written backwards gives 'sllop'.
'sllop' going around 'ca' is 'SCALLOP'.
'in' is the link.
(Other definitions for scallop that I've seen before include "Shellfish - pilgrim badge" , "Shell - sign of a pilgrim" , "Bivalve with radiating ridges on shell" , "Shellfish - badge worn by pilgrims to Santiago de Compostela" , "Marine mollusc" .)