Everyone is on board to make major bet (3,2)
I believe the answer is:
all in
'on board to make major bet' is the definition.
I can't tell whether this defines the answer.
'everyone' is the wordplay.
I cannot really understand how this works, but
'everyone' could be 'all' (all people) and 'all' is found in the answer.
The remaining letters 'in' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'is' acts as a link.
Can you help me to learn more?
(Other definitions for all in that I've seen before include "With everything included" , "What's spent" , "At an inclusive price; exhausted" , "Utterly exhausted - everyone home" , "Everyone's home - exhausted" .)