As king, was he on a perch? (5)
I believe the answer is:
herod
'as king was he on a perch?' is the definition.
The definition suggests a singular noun which matches the answer.
'he on a perch?' is the wordplay.
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'a perch?' becomes 'rod' ('rod' can be a synonym of 'perch').
'he'+'rod'='HEROD'
Can you help me to learn more?
(Other definitions for herod that I've seen before include "child murderer" , "Ruler of Palestine when Jesus was born" , "Cruel king" , "Former king" , "Royal baby-killer" .)