Lock large aircraft door that's not opened (5)

I believe the answer is:
latch
'lock' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for latch that I've seen before include "Door catch" , "Something to secure" , "closer" , "Gate fastener" , "Gate bar" .)
