Politician rested in bed for illness ... (9)
I believe the answer is:
complaint
'illness' is the definition.
(thesaurus)
'politician rested in bed' is the wordplay.
'politician' becomes 'mp'.
'rested' becomes 'lain' (lying is a kind of resting).
'in' indicates putting letters inside.
'bed' becomes 'cot' (cot is a kind of bed).
'mp'+'lain'='mplain'
'mplain' placed inside 'cot' is 'COMPLAINT'.
'for' is the link.
(Other definitions for complaint that I've seen before include "A disease" , "Minor ailment" , "ill-feeling" , "Grievance; illness" , "Indisposition" .)