It's true, many are in this field of activity (5)
I believe the answer is:
realm
'this field of activity' is the definition.
(I've seen this before)
'it's true many' is the wordplay.
'it's true' becomes 'real' (similar in meaning).
'many' becomes 'm' (Roman numeral for a thousand).
'real'+'m'='REALM'
'are in' is the link.
(Other definitions for realm that I've seen before include "World" , "A Nation" , "Monarchy" , "Empire - state" , "Peers are possessed by this" .)