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' says to put letters next to each other.
'bad' is an anagram indicator.
'rot' is an anagram of '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" .)