Disease has politician stretched out flat in bed (9)
I believe the answer is:
complaint
'disease' is the definition.
(medical complaint)
'politician stretched out flat in bed' is the wordplay.
'politician' becomes 'MP' (Member of Parliament).
'stretched out flat' becomes 'lain' (laying down).
'in' is an insertion indicator.
'bed' becomes 'cot' (cot is a kind of bed).
'mp'+'lain'='mplain'
'mplain' placed inside 'cot' is 'COMPLAINT'.
'has' acts as a link.
(Other definitions for complaint that I've seen before include "Sickness" , "Protest" , "Illness; gripe" , "grouse" , "A disease" .)