This setter's stuff is irrelevant (10)
I believe the answer is:
immaterial
'irrelevant' is the definition.
('immaterial' can be similar in meaning to 'irrelevant')
'this setter's stuff' is the wordplay.
'this setter's' becomes 'im' (I've seen this in other clues).
'stuff' becomes 'material' ('material' can be a synonym of 'stuff'**).
'im'+'material'='IMMATERIAL'
'is' is the link.
(Other definitions for immaterial that I've seen before include "Irrelevant" , "unimportant" , "of no importance" .)