Speak sharply or quietly answer back (4)
I believe the answer is:
snap
'speak sharply' is the definition.
(synonyms)
'quietly answer back' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'answer' becomes 'ans' (abbreviation for answer).
'back' is a reversal indicator.
'p'+'ans'='pans'
'pans' written backwards gives 'SNAP'.
'or' is the link.
(Other definitions for snap that I've seen before include "Sudden break" , "Break like informal photo" , "you may take on holiday" , "Break under strain" , "Break - children's card game" .)