Enlist everyone in competition (4,2)
I believe the answer is:
call up
'enlist' is the definition.
(thesaurus)
'everyone in competition' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'in' means one lot of letters goes inside another.
'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 "Remember" , "Ring" , "compulsory service" , "Summon; evoke" , "Order to report for military duty" .)