Rat at home with staff in castle (9)
I believe the answer is:
informant
'rat' is the definition.
(I know that rat can be written as informant)
'home with staff in castle' is the wordplay.
'home' becomes 'in' ('he's home' can mean 'he's in').
'with' is a charade indicator (letters next to each other).
'staff' becomes 'man' (as in staffing or manning a particular place).
'in' is an insertion indicator.
'castle' becomes 'fort' (fort is a type of castle).
'man' placed into 'fort' is 'formant'.
'in'+'formant'='INFORMANT'
'at' acts as a link.
(Other definitions for informant that I've seen before include "Rat" , "Traitor" , "source of dope" , "gives us the gen" , "Snitch" .)