The fellows agree in mind (8)
I believe the answer is:
mentally
'the fellows agree in mind' is the definition.
I can't tell whether this defines the answer.
'the fellows agree' is the wordplay.
'the fellows' becomes 'men' (fellow can mean a man).
'agree' becomes 'tally' (synonyms).
'men'+'tally'='MENTALLY'
Can you help me to learn more?
(Other definitions for mentally that I've seen before include "by way of intellect" , "not in writing" , "not physically" , "in the head" , "Intellectually" .)