... guys agree, it's in the mind (8)
I believe the answer is:
mentally
'in the mind' is the definition.
(I've seen this before)
'guys agree' is the wordplay.
'guys' becomes 'men'.
'agree' becomes 'tally' ('tally' can be a synonym of 'agree').
'men'+'tally'='MENTALLY'
'it's' is the link.
(Other definitions for mentally that I've seen before include "without pencil and paper" , "by way of intellect" , "rationally" , "Cerebrally" , "not in writing" .)