Fellows chat endlessly — it's all in the mind (6)
I believe the answer is:
mental
'it's all in the mind' is the definition.
'fellows chat endlessly' is the wordplay.
'fellows' becomes 'men' (I've seen this before).
'chat' becomes 'talk' (I've seen this before).
'endlessly' means to remove the last letter.
'talk' with its last letter taken away is 'tal'.
'men'+'tal'='MENTAL'
(Other definitions for mental that I've seen before include "Done in the mind" , "Involving the mind" , "Crazy" , "off trolley" , "Cognitive" .)