Field of genuine moss, initially (5)
I believe the answer is:
realm
'field' is the definition.
(I know that realm is a type of field)
'genuine moss initially' is the wordplay.
'genuine' becomes 'real' (associated in meaning).
'initially' indicates taking the first letters.
The first letter of 'moss' is 'm'.
'real'+'m'='REALM'
'of' acts as a link.
(Other definitions for realm that I've seen before include "Kingdom; field of interest" , "King's domain" , "Area of knowledge under discussion" , "A Nation" , "Land under a king or queen" .)