Fellows curtailed discussion of the mind (6)
I believe the answer is:
mental
'of the mind' is the definition.
(I've seen this before)
'fellows curtailed discussion' is the wordplay.
'fellows' becomes 'men' (I've seen this before).
'curtailed' means to remove the last letter.
'discussion' becomes 'talk' (talk is a kind of discussion).
'talk' with its final letter taken off is 'tal'.
'men'+'tal'='MENTAL'
(Other definitions for mental that I've seen before include "Cerebral" , "Cognitive" , "Nuts" , "Involving the mind" , "Crazy" .)