The most common and most likely answer to this crossword clue is the 10 letter word COASTGUARD
C
O
A
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More clues leading to the result COASTGUARD
- Emergency service got us a card, surprisingly
- They rescue you, reportedly trapped by retired bore on cruise
- On cruise, this paper, not the Scotsman, can be a lifesaver
- Drag a scout free — this rescue worker might
- Main force burst gas out: that’s in character
- Got us a card at sea showing a maritime force
- One providing security in the main?
- How Spooner would describe spectral character’s main body?
- Body monitoring offshore activity on Spooner’s phantom Amex?
- CO at a drugs bust?
Buddy explains!

The straight part
We know that "Main defenders" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case main defenders = coastguard.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 COASTGUARD mean?
COASTGUARD noun- The organisation or officer enforcing maritime law and policing the seas within territorial waters.