Romeo shut in huge naval slammer (4)
I believe the answer is:
brig
'naval slammer' is the definition.
I can't tell whether this definition defines the answer.
'romeo shut in huge' is the wordplay.
I cannot quite see how this works, but
'romeo' could be 'r' (phonetic alphabet: alpha, bravo, charlie etc.) and 'r' is found within the answer.
'huge' could be 'big' (similar in meaning) and 'big' is found within the leftover letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for brig that I've seen before include "Ship; a ship's jail" , "officer contracted" , "Crew can" , "Sailing-vessel" , "Two-master; army officer (abbr.)" .)