Guard loses head on way in (5)
I believe the answer is:
entry
'way in' is the definition.
(I've seen this before)
'guard loses head' is the wordplay.
'guard' becomes 'sentry' (sentry is a kind of guard).
'loses head' means to remove the first letter.
'sentry' with its first letter removed is 'ENTRY'.
'on' acts as a link.
(Other definitions for entry that I've seen before include "List of competitors" , "number competing" , "all runners" , "Lobby" , "One in competition" .)