Prompt - and visible, by the sound of it (6)
I believe the answer is:
incite
'prompt' is the definition.
(I know that prompt can be written as incite)
'visible by the sound of it' is the wordplay.
'visible' becomes 'insight' (something in sight is visible).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'insight' sounds like 'INCITE'.
'and' is the link.
(Other definitions for incite that I've seen before include "Foment or arouse" , "Provoke - stimulate" , "Trigger" , "Encourage or stir up" , "Stir up, urge on to action" .)