Naughty child allowed outside for shellfish (6)
I believe the answer is:
limpet
'for 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' is an insertion indicator (some letters go outside others).
'imp' going inside 'let' is 'LIMPET'.
(Other definitions for limpet that I've seen before include "Rock lover" , "clingy type" , "sticker?" , "A shellfish that sticks fast" , "Clinging gastropod" .)