Telephone everyone entering the competition (4,2)
I believe the answer is:
call up
'telephone' is the definition.
('call up' can be a synonym of 'telephone')
'everyone entering the competition' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'entering' indicates putting letters inside.
'the competition' becomes 'cup' (I've seen this before).
'all' inserted into 'cup' is 'CALL-UP'.
(Other definitions for call up that I've seen before include "Remember" , "Order to report for military duty" , "Ring" , "Act of conscription into the armed forces" , "Summon (esp. to army)" .)