Boundless optimism in speech (4)
I believe the answer is:
oral
'speech' is the definition.
(I've seen this before)
'boundless optimism' is the wordplay.
'boundless' says to take the centre.
'optimism' becomes 'morale' (I've seen this before).
The middle of 'morale' is 'ORAL'.
'in' acts as a link.
(Other definitions for oral that I've seen before include "Spoken rather than written" , "Spoken exam" , "from the trap" , "Relating to speech" , "Kind of examination" .)