Right inside huge prison (4)
I believe the answer is:
brig
'prison' is the definition.
(I know that brig is a type of military prison)
'right inside huge' is the wordplay.
'right' becomes 'r' (abbreviation).
'inside' means one lot of letters goes inside another.
'huge' becomes 'big' (similar in meaning).
'r' placed within 'big' is 'BRIG'.
(Other definitions for brig that I've seen before include "senior officer briefly used" , "'Two-masted,square-rigged ship (4)'" , "Ship; ship's prison" , "officer contracted" , "Ship; a ship's jail" .)