No-one takes part in offensive (7)
I believe the answer is:
noisome
'offensive' is the definition.
(I know that offensive can be written as noisome)
'no-one takes part' is the wordplay.
'one' becomes 'i' (Roman numeral).
'takes' means one lot of letters go next to another.
'part' becomes 'some' (I've seen this before).
'no'+'i'+'some'='NOISOME'
'in' acts as a link.
(Other definitions for noisome that I've seen before include "Harmful - offensive" , "Foul; putrid" , "Foul-smelling, noxious" , "Offensive, noxious" , "Smelling bad" .)