The most common and most likely answer to this crossword clue is the 2 plus 3 letter word DO ONE
I've seen this crossword clue in The Times
D
O
O
N
E
More clues leading to the result DO ONE
Buddy explains!

The straight part
We know that "to blow top" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to blow top = do one.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.