The most common and most likely answer to this crossword clue is the 5 plus 4 letter word SUGAR SNAP
I've seen this crossword clue in The Times
S
U
G
A
R
S
N
A
P
More clues leading to the result SUGAR SNAP
Buddy explains!

The straight part
We know that "mangetout" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case mangetout = sugar snap.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.