Naughty child allowed outside for shellfish (6)
I believe the answer is:
limpet
'shellfish' is the definition.
(limpet is a kind of shellfish)
'naughty child allowed outside' is the wordplay.
'naughty child' becomes 'imp' (informal term for a misbehaving child).
'allowed' becomes 'let' (to let is to allow or permit).
'outside' indicates putting letters inside (some letters go outside others).
'imp' put inside 'let' is 'LIMPET'.
'for' is the link.
(Other definitions for limpet that I've seen before include "Rock lover" , "clingy type" , "sticker?" , "A shellfish that sticks fast" , "Clinging gastropod" .)