Intellectual chaps chatter endlessly (6)
I believe the answer is:
mental
'intellectual' is the definition.
(similar in meaning)
'chaps chatter endlessly' is the wordplay.
'chaps' becomes 'men' (chap means a man).
'chatter' becomes 'talk' (I've seen this before).
'endlessly' means to remove the last letter.
'talk' with its last letter taken off is 'tal'.
'men'+'tal'='MENTAL'
(Other definitions for mental that I've seen before include "Intellectual" , "Crazy" , "Light allowing vehicles to turn while ahead is red" , "Done in the mind" , "off trolley" .)