Lie in bed, poorly — it's impossible to get down (8)
I believe the answer is:
inedible
'impossible to get down' is the definition.
(to get something down is to eat it)
'lie in bed poorly' is the wordplay.
'poorly' indicates anagramming the letters.
'lie'+'in'+'bed'='lieinbed'
'lieinbed' with letters rearranged gives 'INEDIBLE'.
'it's' acts as a link.
(Other definitions for inedible that I've seen before include "Not fit for eating" , "Unfit for consumption" , "not to be swallowed" , "Awful to taste" , "Not suitable for consumption" .)