Guard posted home by the Spanish (8)
I believe the answer is:
sentinel
'guard' is the definition.
(sentinel is a kind of guard)
'posted home by the spanish' is the wordplay.
'posted' becomes 'sent' ('send' can be a synonym of 'post').
'home' becomes 'in' ('he's home' can mean 'he's in').
'by' is a charade indicator (letters next to each other).
'the spanish' becomes 'el' ('the' in Spanish).
'sent'+'in'+'el'='SENTINEL'
(Other definitions for sentinel that I've seen before include "Watcher on guard" , "Guardsman" , "Soldier keeping watch" , "Look out" , "One watches" .)