The most common and most likely answer to this crossword clue is the 5 plus 2 plus 5 letter word TOURS DE FORCE
I've seen this crossword clue in Independent
T
O
U
R
S
D
E
F
O
R
C
E
Buddy explains!
The straight part
We know that "Great achievements" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case great achievements = tours de force.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.