A noisy tune is reasonable (4)
I believe the answer is:
fair
'reasonable' is the definition.
('fair' can be a synonym of 'reasonable')
'a noisy tune' is the wordplay.
'a noisy' becomes 'f' (**).
'tune' becomes 'air' (I've seen this before).
'f'+'air'='FAIR'
'is' acts as a link.
(Other definitions for fair that I've seen before include "Just; gala" , "entertainment provided here" , "Having light-coloured hair and skin" , "Light; entertainment" , "promotional exhibition" .)