A loud reasonable matter (6)
I believe the answer is:
affair
'matter' is the definition.
(I know that matter can be written as affair)
'a loud reasonable' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'reasonable' becomes 'fair' (associated in meaning).
'a'+'f'+'fair'='AFFAIR'
(Other definitions for affair that I've seen before include "Liaison or intrigue" , "scandal" , "Matter or concern, perhaps of the heart" , "Secret sexual relationship" , "Romantic relationship between two unmarried people" .)