The most common and most likely answer to this crossword clue is the 3 plus 2 letter word ALL IN
I've seen this crossword clue in The Times
A
L
L
I
N
More clues leading to the result ALL IN
- Ready to drop everything at work?
- Dead-beat | comprehensive
- Margins lost, Estonian capital's drained
- Collapsing out of bounds, exhausted
- Everyone’s entered for the wrestling match
- Spent nothing out, presumably
- Pooped nothing out then?
- Exhausted everything included
- Exhausted, collapse after opener’s dismissed
- Section of vital links wiped out
Buddy explains!

The straight part
We know that "drained" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case drained = all in.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does ALL IN mean?
ALL IN noun- A hand where at least one player bets all of his or her chips.
- A player who is all in.
- (somewhat obsolete) Very tired.
- With everything included.
- Having no further stake to wager, but remaining active in a hand.
- Alternative form of all in