Great man is tense when speaking (5)
I believe the answer is:
titan
'great man' is the definition.
(I've seen this before)
'tense when speaking' is the wordplay.
'tense' becomes 'tighten' (I've seen this before).
'when speaking' shows a homophone (sound like).
'tighten' sounds like 'TITAN'.
'is' acts as a link.
(Other definitions for titan that I've seen before include "One of elder gods defeated by Zeus" , "one of great importance" , "Huge character" , "An old Greek god - Atlas was one" , "Person of great importance" .)