Forest officer called by monarch (6)
I believe the answer is:
ranger
'forest officer' is the definition.
(I've seen this before)
'called by monarch' is the wordplay.
'called' becomes 'rang' ('ring' can be a synonym of 'call').
'by monarch' becomes 'er' (abbreviation for Elizabeth Regina, British Queen. I am not sure about the 'by' bit.).
'rang'+'er'='RANGER'
Can you help me to learn more?
(Other definitions for ranger that I've seen before include "Park warden" , "Forest defender" , "park-keeper" , "Forest or park officer" , "National park officer" .)