The most common and most likely answer to this crossword clue is the 4 plus 3 plus 2 plus 6 letter word HARD ACT TO FOLLOW
I've seen this crossword clue in Financial Times
H
A
R
D
A
C
T
T
O
F
O
L
L
O
W
Buddy explains!
The straight part
We know that "presumably not this" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case presumably not this = hard act to follow.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.