These people calculate the total in their heads (8)
I believe the answer is:
mentally
'in their heads' is the definition.
(I've seen this before)
'these people calculate' is the wordplay.
'these people' becomes 'men' (men are people).
'calculate' becomes 'tally' (I've seen this in another clue).
'men'+'tally'='MENTALLY'
'the total' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for mentally that I've seen before include "without pencil and paper" , "rationally" , "not physically" , "Using the mind" , "how intelligence is developed" .)