A very loud, cool thing (6)
I believe the answer is:
affair
'thing' is the definition.
(I know that thing can be written as affair)
'a very loud cool' is the wordplay.
'very loud' becomes 'ff' (musical abbreviation).
'cool' becomes 'air' (verb - give some air to something).
'a'+'ff'+'air'='AFFAIR'
(Other definitions for affair that I've seen before include "Amorous liaison" , "Romantic relationship between two unmarried people" , "Secret sexual relationship" , "Concern,matter" , "A matter, perhaps of love" .)