Edible shellfish in middle of seabed singly (7)
I believe the answer is:
abalone
'edible shellfish' is the definition.
'middle of seabed singly' is the wordplay.
'middle of' means to look at the middle letters.
'singly' becomes 'alone' (I've seen this before).
The middle letters of 'seabed' are 'ab'.
'ab'+'alone'='ABALONE'
'in' is the link.
(Other definitions for abalone that I've seen before include "Edible shellfish, sea-ear" , "something to eat from the sea" , "Mother-of-pearl could have come from" , "Ear-shaped shell" , "gastropod" .)