Party mum laid on in field (6)
I believe the answer is:
domain
'field' is the definition.
('domain' can be a synonym of 'field')
'party mum laid on in' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'mum' becomes 'ma' ('ma' can be a synonym of 'mum').
'laid on' is a charade indicator (letters next to each other).
'do'+'ma'+'in'='DOMAIN'
(Other definitions for domain that I've seen before include "Territory over which someone reigns" , "Kingdom or realm" , "Field of thought or activity" , "District or area - or range of influence" , "Range of influence or field of activity" .)