A noisy tune is reasonable (4)
I believe the answer is:
fair
'reasonable' is the definition.
(synonyms)
'a noisy tune' is the wordplay.
'a noisy' becomes 'f' ().
'tune' becomes 'air' (air can mean a piece of music).
'f'+'air'='FAIR'
'is' is the 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" .)