People agree in thought (8)
I believe the answer is:
mentally
'in thought' is the definition.
(I've seen this in another clue)
'people agree' is the wordplay.
'people' becomes 'men' (men are people).
'agree' becomes 'tally' (synonyms).
'men'+'tally'='MENTALLY'
(Other definitions for mentally that I've seen before include "not physically" , "in the head" , "how intelligence is developed" , "Cerebrally" , "in their heads" .)