"Go," possibly said very softly to a listener (9)
I believe the answer is:
disappear
'go' is the definition.
(I know that go is a more specific form of disappear)
'possibly said very softly to a listener' is the wordplay.
'possibly' indicates anagramming the letters.
'very softly' becomes 'pp' (abbreviation for pianissimo - very quietly).
'to a listener' becomes 'ear' (body part which hears or listens).
'said' anagrammed gives 'disa'.
'disa'+'pp'+'ear'='DISAPPEAR'
(Other definitions for disappear that I've seen before include "go by the board" , "Pass out of sight" , "Melt away" , "Get lost" , "suddenly depart" .)