No one takes part - it's most offensive (7)
I believe the answer is:
'most offensive' is the definition.
'noisome' can be an answer for 'offensive' (similar in meaning). I'm unsure of the 'most' bit.
'no one takes part' is the wordplay.
'one' becomes 'i' (Roman numeral).
'takes' says to put letters next to each other.
'part' becomes 'some' (I've seen this before).
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for noisome that I've seen before include "Extremely unpleasant" , "Smelling bad" , "Evil-smelling - sounds loud" , "noxious smells" , "Offensive, noxious" .)