Secret romance in a very loud atmosphere (6)
I believe the answer is:
affair
'secret romance' is the definition.
(I've seen this before)
'a very loud atmosphere' is the wordplay.
'very loud' becomes 'ff' (musical abbreviation).
'atmosphere' becomes 'air' (synonyms).
'a'+'ff'+'air'='AFFAIR'
'in' is the link.
(Other definitions for affair that I've seen before include "Raffia (anag)" , "Clandestine romance" , "connected with love" , "Matter or concern, perhaps of the heart" , "Concern -- extramarital relationship" .)