The most common and most likely answer to this crossword clue is the 3 plus 3 plus 4 plus 2 plus 2 letter word GOT THE HANG OF IT
I've seen this crossword clue in The Daily Telegraph
G
O
T
T
H
E
H
A
N
G
O
F
I
T
Buddy explains!
The straight part
We know that "Grasped the principle of putting curtains up?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grasped the principle of putting curtains up? = got the hang of it.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.