Guys count in their heads (8)
I believe the answer is:
mentally
'in their heads' is the definition.
(I've seen this before)
'guys count' is the wordplay.
'guys' becomes 'men'.
'count' becomes 'tally' ('tally' can be a synonym of 'count').
'men'+'tally'='MENTALLY'
(Other definitions for mentally that I've seen before include "in the head" , "not physically" , "by way of intellect" , "Cerebrally" , "without pencil and paper" .)