Rat in castle biting servant (9)
I believe the answer is:
informant
'rat' is the definition.
(I know that rat can be written as informant)
'in castle biting servant' is the wordplay.
'castle' becomes 'fort' (fort is a type of castle).
'biting' is an insertion indicator (some letters bite or swallow others).
'servant' becomes 'man' (man is a kind of servant).
'fort' placed around 'man' is 'formant'.
'in'+'formant'='INFORMANT'
(Other definitions for informant that I've seen before include "Snitch" , "Grass" , "Telltale" , "Rat" , "Sneak" .)