The most common and most likely answer to this crossword clue is the 7 letter word CLIMBED
I've seen this crossword clue in The Guardian
C
L
I
M
B
E
D
Buddy explains!

The straight part
We know that "Reached top of" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case reached top of = climbed.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 CLIMBED mean?
CLIMBED verb- To ascend; rise; to go up.
- To mount; to move upwards on.
- To scale; to get to the top of something.
- To move (especially up and down something) by gripping with the hands and using the feet.
- To practise the sport of climbing
- To jump high
- To move to a higher position on the social ladder.
- Of plants, to grow upwards by clinging to something.