Bearing of man who’ll succeed when speaking (3)
I believe the answer is:
air
'bearing' is the definition.
(I've seen this before)
'man who'll succeed when speaking' is the wordplay.
'man who'll succeed' becomes 'heir' (an heir succeeds to a title).
'when speaking' shows a homophone (sound like).
'heir' sounds like 'AIR'.
'of' acts as a link.
(Other definitions for air that I've seen before include "Short song" , "Broadcast - song" , "what emerges from singer's lungs?" , "Ventilate or give expression to" , "piece of music" .)