Lethargic duck turned around behind hill (6)
I believe the answer is:
torpid
'lethargic' is the definition.
(synonyms)
'duck turned around behind hill' is the wordplay.
'duck' becomes 'dip' (both can mean to plunge into water).
'turned around' says the letters should be written backwards.
'behind' is a charade indicator (letters next to each other).
'hill' becomes 'tor' (tor is a kind of hill).
'dip' back-to-front is 'pid'.
'pid' put after 'tor' is 'TORPID'.
(Other definitions for torpid that I've seen before include "Lethargic, sluggish" , "hibernating?" , "Slow and apathetic" , "Slow, dull" , "Sluggish and inactive" .)