The most common and most likely answer to this crossword clue is the 3 letter word TOO
I've seen this crossword clue in The Times
T
O
O
More clues leading to the result TOO
- Also accepted king must go
- Likewise at solo, ignoring the odds
- Hammer possibly left out as well
- Overheard couple too much
- Old books brought up in addition
- Sort of square rings on top of that
- Rounds on warning from motorist that’s oversentimental
- Leaders in The Oval Office, also
- In addition, a couple speaking
- Very good retiring from a hot country
Buddy explains!

The straight part
We know that "on top of that" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case on top of that = too.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 TOO mean?
TOO adverb- (focus) Likewise.
- Also; in addition.
- (degree) To an excessive degree; over; more than enough.
- (degree) To a high degree, very.
- (affirmation) Used to contradict a negative assertion.