Allow a devil in and he's not easily shifted (6)
I believe the answer is:
limpet
'he's not easily shifted' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'allow a devil in' is the wordplay.
'allow' becomes 'let' (synonyms).
'a devil' becomes 'imp' (devilish creature).
'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 "It clings to rock surfaces" , "A shellfish that sticks fast" , "Proverbially clingy mollusc" , "Marine mollusc" , "Mollusc adhering to rocks" .)