Rogue beset by obstruction that's difficult to shift (6)
I believe the answer is:
limpet
'that's difficult to shift' is the definition.
(mollusc which attaches firmly to surfaces)
'rogue beset by obstruction' is the wordplay.
'rogue' becomes 'imp'.
'beset by' is an insertion indicator ('beset' could historically mean 'cover').
'obstruction' becomes 'let' (I've seen this in other clues).
'imp' placed within 'let' is 'LIMPET'.
(Other definitions for limpet that I've seen before include "Sea creature (or person) not easily shifted" , "Clingy person" , "Mollusc adhering to rocks" , "A shellfish that sticks to rocks or ships" , "he's not easily got rid of" .)