Romeo on the outside is one that's horny and miserable (6)
I believe the answer is:
morose
'miserable' is the definition.
(I've seen this before)
'romeo on the outside is one' is the wordplay.
'romeo on' becomes 'moro' (I can't explain this - if you can you should believe this answer much more).
'the outside' suggests removing the centre (I've seen 'outside of' mean this).
'is one' becomes 'same' (similar in meaning).
'same' with its centre taken out is 'se'.
'moro'+'se'='MOROSE'
'that's horny and' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for morose that I've seen before include "dark" , "Sullen, crabby" , "Sullen and moody" , "Surly and unforthcoming" , "Sullen or glum." .)