Allow little devil in and he's hard to shift (6)
I believe the answer is:
limpet
'he's hard to shift' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'allow little devil in' is the wordplay.
'allow' becomes 'let' (synonyms).
'little devil' becomes 'imp' (an imp is a little demon).
'in' is an insertion indicator.
'let' going around 'imp' is 'LIMPET'.
'and' acts as a link.
Can you help me to learn more?
(Other definitions for limpet that I've seen before include "Marine creature clinging to a rock" , "clingy type" , "A shellfish that sticks to rocks or ships" , "Clinging gastropod" , "Sea creature (or person) not easily shifted" .)