The most common and most likely answer to this crossword clue is the 7 letter word RUMBLED
I've seen this crossword clue in The Times
R
U
M
B
L
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D
More clues leading to the result RUMBLED
Buddy explains!

The straight part
We know that "Was onto" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case was onto = rumbled.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 RUMBLED mean?
RUMBLED verb- To make a low, heavy, continuous sound.
- To discover deceitful or underhanded behaviour.
- To move while making a rumbling noise.
- To fight; to brawl.
- (of a game controller) to provide haptic feedback by vibrating.
- To cause to pass through a rumble, or polishing machine.
- To murmur; to ripple.