Telephone everyone in competition (4,2)
I believe the answer is:
call up
'telephone' is the definition.
('call up' can be a synonym of 'telephone')
'everyone in competition' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'in' indicates putting letters inside.
'competition' becomes 'cup' (I've seen this before).
'all' placed inside 'cup' is 'CALL-UP'.
(Other definitions for call up that I've seen before include "Summons for military service" , "enlist" , "Act of conscription into the armed forces" , "Summons to serve in the armed forces" , "Summon; evoke" .)