The soldier posted to keep watch lost his head at the gate (5)
I believe the answer is:
entry
'the gate' is the definition.
(I've seen this before)
'the soldier posted to keep watch lost his head' is the wordplay.
'the soldier posted to keep watch' becomes 'sentry' (I've seen this before).
'lost his head' means to remove the first letter.
'sentry' with its initial letter taken away is 'ENTRY'.
'at' acts as a link.
(Other definitions for entry that I've seen before include "One in competition" , "List of competitors" , "Record - number of contestants" , "all runners" , "List of candidates" .)