The most common and most likely answer to this crossword clue is the 8 letter word TUBEROSE
I've seen this crossword clue in The Times
T
U
B
E
R
O
S
E
More clues leading to the result TUBEROSE
Buddy explains!
The straight part
We know that "with knobs" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with knobs = tuberose.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 TUBEROSE mean?
TUBEROSE noun- A Mexican tuberous plant (Polianthes tuberosa) that has white flowers and grass-like leaves, used in perfumery.
- Tuberous