Live quartet heard previously (6)
I believe the answer is:
before
'previously' is the definition.
('before' can be a synonym of 'previously')
'live quartet heard' is the wordplay.
'live' becomes 'be' ('be' can be a synonym of 'live').
'quartet' becomes 'four' ('four' can be a synonym of 'quartet').
'heard' indicates a 'sounds like' (homophone) clue.
'four' is a homophone of 'fore'.
'be'+'fore'='BEFORE'
(Other definitions for before that I've seen before include "On a previous occasion" , "In front of (something)" , "In front of or previous to" , "During the period preceding" , "Earlier in time" .)