Party leaders to mess around in field (6)
I believe the answer is:
domain
'field' is the definition.
(I know that field can be written as domain)
'party leaders to mess around in' is the wordplay.
'party' becomes 'do' (do is a kind of party).
'leaders to' suggests taking the first letters (I've seen 'leader to' mean this (the letter which leads the word)).
The initial letters of 'mess around' is 'ma'.
'do'+'ma'+'in'='DOMAIN'
(Other definitions for domain that I've seen before include "estate" , "Area under someone's control, field of activity" , "Field of thought or activity" , "Sphere of activity" , "Sphere or territory" .)