Summon everybody to enter competition (4,2)
I believe the answer is:
call up
'summon' is the definition.
(I know that summon can be written as call up)
'everybody to enter competition' is the wordplay.
'everybody' becomes 'all' (I've seen this before).
'to enter' indicates putting letters inside.
'competition' becomes 'cup' (I've seen this before).
'all' inserted within 'cup' is 'CALL-UP'.
(Other definitions for call up that I've seen before include "Act of conscription into the armed forces" , "Summons for military service" , "compulsory service" , "Summons to serve in the armed forces" , "Summon (esp. to army)" .)