Sank hull of icebreaker in sea (5)
I believe the answer is:
mired
'sank' is the definition.
(sank in mud or mire)
'hull of icebreaker in sea' is the wordplay.
'hull of icebreaker in' becomes 'mi' (I can't explain this - if you can you should believe this answer much more).
'sea' becomes 'red' (the Red Sea).
'mi'+'red'='MIRED'
Can you help me to learn more?
(Other definitions for mired that I've seen before include "Stuck in the mud" , "covered in muck" , "Sunk in deep mud" , "Bogged down" .)