'Number One', a little offensive (7)
I believe the answer is:
noisome
'offensive' is the definition.
('noisome' can be similar in meaning to 'offensive')
'number one a little' is the wordplay.
'number' becomes 'no' (abbreviation for 'number').
'one' becomes 'i' (Roman numeral).
'a little' becomes 'some' (I've seen this before).
'no'+'i'+'some'='NOISOME'
(Other definitions for noisome that I've seen before include "Disagreeable - noxious" , "Extremely unpleasant" , "Offensive, noxious" , "Smelling bad" , "Offensively malodorous" .)