Shellfish allowed to swallow naughty child (6)
I believe the answer is:
limpet
'shellfish' is the definition.
(I know that limpet is a type of shellfish)
'allowed to swallow naughty child' is the wordplay.
'allowed to' becomes 'let' (to let something happen is to allow it).
'swallow' indicates putting letters inside.
'naughty child' becomes 'imp' (informal term for a misbehaving child).
'let' placed around 'imp' is 'LIMPET'.
(Other definitions for limpet that I've seen before include "Shellfish that sticks to surface" , "he's not easily got rid of" , "Clinging gastropod" , "Proverbially clinging shellfish" , "sticker?" .)