One member's allowed out for shellfish (6)
I believe the answer is:
limpet
'shellfish' is the definition.
(limpet is a kind of shellfish)
'one member's allowed out' is the wordplay.
'one' becomes 'i' (Roman numeral).
'member' becomes 'MP' (Member of Parliament).
'allowed' becomes 'let' (to let is to allow or permit).
'out' means one lot of letters goes inside another.
'i'+'mp'='imp'
'imp' inserted within 'let' is 'LIMPET'.
'for' is the link.
(Other definitions for limpet that I've seen before include "clingy type" , "Adhesive mollusc" , "Clingy person" , "Clinging gastropod" , "he's not easily got rid of" .)