Rumour about old man is disgusting (7)
I believe the answer is:
noisome
'is disgusting' is the definition.
(I've seen this before)
'rumour about old man' is the wordplay.
'rumour' becomes 'noise' (I've seen this before).
'about' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'man' becomes 'm'.
'o'+'m'='om'
'noise' placed around 'om' is 'NOISOME'.
(Other definitions for noisome that I've seen before include "Odorous" , "Nauseating" , "Offensively smelling" , "Smelling badly" , "Odious" .)