Left the mare cantering round the field (5)
I believe the answer is:
realm
'the field' is the definition.
('realm' can be a synonym of 'field')
'left the mare cantering round' is the wordplay.
'left' becomes 'L' (common abbreviation).
'cantering round' is an anagram indicator.
'l'+'mare'='lmare'
'lmare' anagrammed gives 'REALM'.
(Other definitions for realm that I've seen before include "Kingdom or sphere" , "region" , "Province" , "Kingdom, domain" , "Royal domain" .)