The guard has been posted: Neil Turner (8)
I believe the answer is:
sentinel
'the guard' is the definition.
(sentinel is a kind of guard)
'posted neil turner' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'turner' indicates anagramming the letters (letters must turn into another order).
'neil' with letters rearranged gives 'inel'.
'sent'+'inel'='SENTINEL'
'has been' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for sentinel that I've seen before include "One on guard" , "Watchperson" , "Sentry" , "One watching" , "One watches" .)