The most common and most likely answer to this crossword clue is the 6 plus 7 letter word ROCKET SCIENCE
I've seen this crossword clue in Financial Times
R
O
C
K
E
T
S
C
I
E
N
C
E
More clues leading to the result ROCKET SCIENCE
- Clever stuff investing energy and time in geology, say?
- Music scene etc I abandoned for intellectually demanding activity
- Eccentric old English king, upset about son? That’s a very tricky subject!
- The cultivation and care of arugula could be an intellectually demanding activity
- Something intellectually challenging that goes beyond blue-sky thinking?
- Brain surgery that’s required by astronaut?
- Clever stuff investing energy and time in geology?
Buddy explains!
The straight part
We know that "very tricky subject" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case very tricky subject = rocket science.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.