Let the devil in, it’s mine (6)
I believe the answer is:
limpet
'mine' is the definition.
(I've seen this before)
'let the devil in' is the wordplay.
'the devil' becomes 'imp' (devilish creature).
'in' indicates putting letters inside.
'let' placed around 'imp' is 'LIMPET'.
'it's' acts as a link.
(Other definitions for limpet that I've seen before include "Mollusc adhering to rocks" , "Shellfish that sticks to surface" , "Cinging mollusc" , "A shellfish that sticks to rocks or ships" , "It clings to rock surfaces" .)