Restroom at sea, hearty! (10)
I believe the answer is:
headstrong
'restroom' is the definition.
I can't tell whether this definition defines the answer.
'sea hearty' is the wordplay.
'sea' becomes 'head' (I can't explain this - if you can you should give a lot more credence to this answer).
'hearty' becomes 'strong' (I've seen this in another clue).
'head'+'strong'='HEADSTRONG'
'at' acts as a link.
Can you help me to learn more?
(Other definitions for headstrong that I've seen before include "pig-headed" , "Wilful, determined" , "Stubborn" , "Obstinate" , "strong-willed" .)