The most common and most likely answer to this crossword clue is the 17 letter word TRUDGING/INGULFED
T
R
U
D
G
I
N
G
/
I
N
G
U
L
F
E
D
Buddy explains!
The straight part
We know that "making slow progress" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case making slow progress = trudging/ingulfed.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.