Everyone taking part completely exhausted (3,2)
I believe the answer is:
all in
'completely exhausted' is the definition.
(I've seen this before)
'everyone taking part' is the wordplay.
'everyone' becomes 'all' (I've seen this before).
'taking' is a charade indicator (letters next to each other).
'part' becomes 'in' ('in part' can be a synonym of 'part').
'all'+'in'='ALL-IN'
(Other definitions for all in that I've seen before include "With everything included" , "At an inclusive price; exhausted" , "Worn-out" , "Including everything" , "Inclusive; tired" .)