The most common and most likely answer to this crossword clue is the 5 letter word ALIBI
I've seen this crossword clue in The Times
A
L
I
B
I
More clues leading to the result ALIBI
- A political party member, one making excuse
- Making excuse, do cut off a sexual urge
- Politician protected by first-class defence
- One in England missing out on defence
- Fighter starts to box intelligently, presenting defence
- Excuse one replacing a graduate in panto role
- Excuse a former politician invented originally
- Plea from Somali bishop
- A politician of yesteryear, one to offer defence
- Provide excuse for a sexual impulse — finish being banned
Buddy explains!

The straight part
We know that "defence" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case defence = alibi.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 ALIBI mean?
ALIBI noun- The plea or mode of defense under which a person on trial for a crime proves or attempts to prove being in another place when the alleged act was committed
- To provide an alibi for.
- To provide an excuse for.