Herb's occasion to be heard (5)
I believe the answer is:
thyme
'herb's' is the definition.
(thyme is a kind of herb)
'occasion to be heard' is the wordplay.
'occasion' becomes 'time' (I've seen this before).
'to be heard' shows a homophone (sound like).
'time' is a homophone of 'THYME'.
(Other definitions for thyme that I've seen before include "Minty herb" , "Culinary herb of the mint family" , "Common herb" , "Herb used in cooking as seasoning" , "Aromatic shrub" .)