The most common and most likely answer to this crossword clue is the 8 letter word KOWTOWED
I've seen this crossword clue in The Sunday Telegraph
K
O
W
T
O
W
E
D
Buddy explains!

The straight part
We know that "demonstrated subservience" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case demonstrated subservience = kowtowed.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 KOWTOWED mean?
KOWTOWED verb- To grovel, act in a very submissive manner.
- To kneel and bow low enough to touch one’s forehead to the ground.
- To bow very deeply.