The most common and most likely answer to this crossword clue is the 7 letter word PLEDGEE
I've seen this crossword clue in The Guardian
P
L
E
D
G
E
E
Buddy explains!
The straight part
We know that "One given an assurance" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one given an assurance = pledgee.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 PLEDGEE mean?
PLEDGEE noun- Someone who receives a pledge