Way in which the sentry loses his head (5)
I believe the answer is:
entry
'way in' is the definition.
(I've seen this before)
'the sentry loses his head' is the wordplay.
'loses his head' means to remove the first letter.
'sentry' with its first letter removed is 'ENTRY'.
'which' is the link.
(Other definitions for entry that I've seen before include "The act of going in" , "One in competition" , "Admission" , "Item in an account book" , "List of candidates" .)