Summon from top to bottom, King gives green signal (3,5)
I believe the answer is:
all clear
'signal' is the definition.
(all clear is a kind of signal)
'summon from top' is the wordplay.
'summon from' becomes 'all' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'top' becomes 'clear' (synonyms).
'all'+'clear'='ALL-CLEAR'
'to bottom king gives green' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for all clear that I've seen before include "old signal to get out of bunker?" , "Siren sound to mark end of bombing by planes" , "Permission to proceed" , "Signal that it's safe to come out" , "no doubts" .)