Encircling chief - he's no royalist (9)
I believe the answer is:
roundhead
'he's no royalist' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'encircling chief' is the wordplay.
'encircling' becomes 'round' (I've seen this before).
'chief' becomes 'head' (synonyms).
'round'+'head'='ROUNDHEAD'
Can you help me to learn more?
(Other definitions for roundhead that I've seen before include "antimonarchist" , "Parliamentarian (hist.)" , "Supporter of Parliament during the English Civil Wars" , "Old soldier" , "An enemy of Charles" .)