Get in the pool (3)
I believe the answer is:
bet
'get in the pool' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for bet that I've seen before include "'Stake, wager (3)'" , "Wager, gamble" , "Wager, back" , "22 across will accept one" , "Wager, or the sum waged" .)