The most common and most likely answer to this crossword clue is the 6 letter word PLEDGE
I've seen this crossword clue in Independent
P
L
E
D
G
E
More clues leading to the result PLEDGE
- Place to slightly shift commitment
- Word for one turning on soft, small light
- Word taken from page shortened book
- Promise to put shelf behind piano
- Word used by unprincipled genius
- Promise shown by piano student on Fringe
- Security needed for place on the border
- Promise place on Fringe
- Promise given by character pursuing first of spinsters on shelf
- Assurance in place with advantage
Buddy explains!

The straight part
We know that "Security" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case security = pledge.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 PLEDGE mean?
PLEDGE noun- A solemn promise to do something.
- A security to guarantee payment of a debt.
- A person who has taken a pledge of allegiance to a college fraternity, but is not yet formally approved.
- A drinking toast.
- To make a solemn promise (to do something).
- To deposit something as a security; to pawn.
- To give assurance of friendship by the act of drinking; to drink to one's health.