Field one inside silly mid-on (6)
I believe the answer is:
domain
'field' is the definition.
('domain' can be a synonym of 'field')
'one inside silly mid-on' is the wordplay.
'one' becomes 'a'.
'inside' indicates putting letters inside.
'silly' is an anagram indicator.
'mid'+'on'='midon'
'midon' anagrammed gives 'domin'.
'a' going into 'domin' is 'DOMAIN'.
(Other definitions for domain that I've seen before include "Sphere of activity" , "Realm or range of influence" , "Territory over which rule or control is exercised" , "Special region, area of expertise" , "Scope of knowledge - owned land" .)