Clingy sort of scamp, allowed outside (6)
I believe the answer is:
limpet
'clingy sort' is the definition.
(mollusc which clings to rocks)
'scamp allowed outside' is the wordplay.
'scamp' becomes 'imp' (synonyms).
'allowed' becomes 'let' (I've seen this before).
'outside' means one lot of letters goes inside another.
'imp' put inside 'let' is 'LIMPET'.
'of' acts as a link.
(Other definitions for limpet that I've seen before include "A shellfish that sticks to rocks or ships" , "Adhesive mollusc" , "Clingy person" , "Clinging mollusc" , "Sea creature (or person) not easily shifted" .)