The most common and most likely answer to this crossword clue is the 5 letter word LEGGY
I've seen this crossword clue in The Times
L
E
G
G
Y
More clues leading to the result LEGGY
Buddy explains!

The straight part
We know that "sluggish" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case sluggish = leggy.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.
What does LEGGY mean?
LEGGY adjective- (chiefly of a woman) Having long, attractive legs; long-legged.
- (chiefly of a woman) Exposing the bare or pantyhose-clad legs, especially the thighs.
- Taller or longer than usual.