Overdose promises to be foul (6)
I believe the answer is:
odious
'be foul' is the definition.
(thesaurus)
'overdose promises' is the wordplay.
'overdose' becomes 'o.d.' (synonyms).
'promises' becomes 'IOUs' (promises to pay a debt).
'od'+'ious'='ODIOUS'
'to' acts as a link.
(Other definitions for odious that I've seen before include "Ghastly" , "'Hateful, repulsive (6)'" , "Replusive, abhorrent (6)" , "Extremely unpleasant" , "Hateful, generally despised" .)