The most common and most likely answer to this crossword clue is the 3 letter word MOP
I've seen this crossword clue in Financial Times
M
O
P
More clues leading to the result MOP
Buddy explains!

The straight part
We know that "Cleaner" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case cleaner = mop.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 MOP mean?
MOP noun- An implement for washing floors, or the like, made of a piece of cloth, or a collection of thrums, or coarse yarn, fastened to a handle.
- A wash with a mop; the act of mopping
- A dense head of hair.
- A fair where servants are hired.
- The young of any animal; also, a young girl; a moppet.
- A made-up face; a grimace.
- To rub, scrub, clean or wipe with a mop, or as if with a mop.
- To make a wry expression with the mouth.