Rat popular with staff in castle (9)
I believe the answer is:
informant
'rat' is the definition.
(I know that rat can be written as informant)
'popular with staff in castle' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'with' is a charade indicator (letters next to each other).
'staff' becomes 'man' (as in staffing or manning a particular place).
'in' indicates putting letters inside.
'castle' becomes 'fort' (fort is a type of castle).
'man' inserted within 'fort' is 'formant'.
'in'+'formant'='INFORMANT'
(Other definitions for informant that I've seen before include "One who sings" , "Sneak" , "Telltale" , "gives us the gen" , "source of dope" .)