True leader of men in field (5)
I believe the answer is:
realm
'field' is the definition.
(synonyms)
'true leader of men' is the wordplay.
'true' becomes 'real' (synonyms).
'leader of' says to take the initial letters.
The first letter of 'men' is 'm'.
'real'+'m'='REALM'
'in' is the link.
(Other definitions for realm that I've seen before include "King's domain" , "Dominion" , "Area of knowledge under discussion" , "Monarchy" , "Kingdom or sphere" .)