Right inside massive naval prison (4)
I believe the answer is:
brig
'naval prison' is the definition.
'brig' can be an answer for 'prison' (I've seen this before). I'm not sure about the 'naval' bit.
'right inside massive' is the wordplay.
'right' becomes 'r' (common abbreviation).
'inside' means one lot of letters goes inside another.
'massive' becomes 'big' (similar in meaning).
'r' placed into 'big' is 'BRIG'.
Can you help me to learn more?
(Other definitions for brig that I've seen before include "A ship; army officer (abbr.)" , "Ship; ship's prison" , "Two-masted square-rigged vessel" , "Crew can" , "officer contracted" .)