Sluggish hill descent coming up (6)
I believe the answer is:
torpid
'sluggish' is the definition.
(synonyms)
'hill descent coming up' is the wordplay.
'hill' becomes 'tor' (tor is a kind of hill).
'descent' becomes 'dip' (dip is a kind of descent).
'coming up' is a reversal indicator (in a down clue, letters go up).
'dip' in reverse letter order is 'pid'.
'tor'+'pid'='TORPID'
(Other definitions for torpid that I've seen before include "Conservative nearly given money to withhold article" , "Physically inactive" , "Lethargic - drop it (anag)" , "Inactive, dormant" , "Slow, dull" .)