The wintry sea can be a discouragement (4,5)
I believe the answer is:
cold water
'be a discouragement' is the definition.
The definition suggests a singular noun which matches the answer.
'the wintry sea' is the wordplay.
'the wintry' becomes 'cold' (I've seen this in another clue).
'sea' becomes 'water' (I can't explain this - if you can you should believe this answer much more).
'cold'+'water'='COLD WATER'
'can' acts as a link.
Can you help me to learn more?
(Other definitions for cold water that I've seen before include "It's discouraging stuff to pour" , "Throw this on idea to be discouraging" , "Throw this on plan to discourage or rubbish it" , "Throw this on idea to dampen enthusiasm" , "Pour . . . . . . . . . on something to be discouraging" .)