The most common and most likely answer to this crossword clue is the 6 letter word TORPID
I've seen this crossword clue in The Sunday Telegraph
T
O
R
P
I
D
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 = torpid.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 TORPID mean?
TORPID noun- (Oxford University slang) An inferior racing boat, or one who rows in such a boat.
- Unmoving
- Dormant or hibernating
- Lazy, lethargic or apathetic