Witness bear feeding beside monarch (9)
I believe the answer is:
bystander
'witness' is the definition.
(bystander is a kind of witness)
'bear feeding beside monarch' is the wordplay.
'bear' becomes 'stand' (synonyms).
'feeding' means one lot of letters goes inside another.
'beside' becomes 'by'.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'by'+'er'='byer'
'stand' put within 'byer' is 'BYSTANDER'.
(Other definitions for bystander that I've seen before include "Non-participant spectator" , "Watcher of occurrence" , "Onlooker (often innocent)" , "Bad sentry may be an onlooker" , "Non-partcipating spectator" .)