Quintet, loud, that is maintaining volume (4)
I believe the answer is:
five
'quintet' is the definition.
(synonyms)
'loud that is maintaining volume' is the wordplay.
'loud' becomes 'f' (short for forte - 'loud' in music).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'maintaining' means one lot of letters goes inside another (inserted letters are held or maintained).
'volume' becomes 'v' (abbreviation for volume in book titles).
'ie' enclosing 'v' is 'ive'.
'f'+'ive'='FIVE'
(Other definitions for five that I've seen before include "Prime number" , "Quintet" , "taken for the rest of the musicians" , "Odd number" , "End of the working day?" .)