Guard decapitated in narrow passage (5)
I believe the answer is:
entry
'narrow passage' is the definition.
'entry' can be an answer for 'passage' (I've seen this in another clue). I'm unsure of the 'narrow' bit.
'guard decapitated' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'decapitated' means to remove the first letter.
'sentry' with its first letter taken away is 'ENTRY'.
'in' acts as a link.
Can you help me to learn more?
(Other definitions for entry that I've seen before include "Opening" , "Admittance" , "Item inserted in a written record" , "Debut" , "Way in; item in list" .)