He advises men on bad rot (6)
I believe the answer is:
mentor
'he advises' is the definition.
(I've seen this before)
'men on bad rot' is the wordplay.
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'bad' indicates anagramming the letters.
'rot' anagrammed gives 'tor'.
'men'+'tor'='MENTOR'
(Other definitions for mentor that I've seen before include "Train" , "A trusted teacher or counsellor" , "Counsellor/guide" , "One guiding" , "Wise and trusted guide and advisor" .)