Rock salt's added to fish, mainly as deal-breaker (4,5)
I believe the answer is:
last straw
'rock salt's' is the definition.
Both the definition and answer are singular nouns.
Perhaps you can see an association between them that I don't see?
'added to fish mainly as deal-breaker' is the wordplay.
'added to' is a charade indicator (letters next to each other).
'fish' becomes 'trawl' (trawl can mean to fish).
'mainly' means to remove the last letter.
'as deal breaker' becomes 'lasts' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'trawl' with its final letter removed is 'traw'.
'traw' put after 'lasts' is 'LAST STRAW'.
Can you help me to learn more?
(Other definitions for last straw that I've seen before include "The final provocation" , "Something that finally makes a situation intolerable" , "ultimate irritation" , "drinker was lucky to get?" , "Back-breaking assignment" .)