Dwelling on me is rotten (4)
I believe the answer is:
semi
'dwelling' is the definition.
(I've seen this before)
'on me is rotten' is the wordplay.
'on' means one lot of letters go next to another.
'rotten' indicates an anagram.
'me' after 'is' is 'isme'.
'isme' is an anagram of 'SEMI'.
(Other definitions for semi that I've seen before include "accommodation" , "House sharing a wall" , "A whole house (or half?)" , "House joined to another" , "One house of pair" .)