The most common and most likely answer to this crossword clue is the 4 plus 5 plus 2 plus 2 letter word JUST THINK OF IT
I've seen this crossword clue in The Times
J
U
S
T
T
H
I
N
K
O
F
I
T
Buddy explains!
The straight part
We know that "Contemplate but don’t act/that shouldn’t happen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case contemplate but don’t act/that shouldn’t happen = just think 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.