Intellectual fellows converse endlessly (6)
I believe the answer is:
mental
'intellectual' is the definition.
('mental' is associated in meaning with 'intellectual')
'fellows converse endlessly' is the wordplay.
'fellows' becomes 'men' (I've seen this before).
'converse' becomes 'talk' (I've seen this before).
'endlessly' means to remove the last letter.
'talk' with its final letter removed is 'tal'.
'men'+'tal'='MENTAL'
(Other definitions for mental that I've seen before include "Lament (anag) - of the mind" , "off trolley" , "Involving the mind" , "like brain operation" , "Cerebral" .)