The most common and most likely answer to this crossword clue is the 9 letter word MUSCLIEST
I've seen this crossword clue in Independent
M
U
S
C
L
I
E
S
T
Buddy explains!

The straight part
We know that "pumped in the extreme!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pumped in the extreme! = muscliest.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.
Crossword Answer definitions
MUSCLIEST adjective- Thewy, muscled
- Strong, powerful