It’s sweet to get fish in tails - nearly throw up! (9)
I believe the answer is:
saccharin
'it's sweet' is the definition.
I can't judge whether this defines the answer.
'to get fish in tails nearly throw up' is the wordplay.
'to get fish' becomes 'char' (I can't explain this - if you can you should believe this answer much more).
'tails' says to put letters next to each other (tail can mean to follow).
'nearly' means to remove the last letter.
'throw' becomes 'cast' (casting is a kind of throwing).
'up' is a reversal indicator (in down clue: letters go upwards).
'cast' with its last letter taken off is 'cas'.
'cas' back-to-front is 'sac'.
'char'+'in'='charin'
'charin' put after 'sac' is 'SACCHARIN'.
Can you help me to learn more?
(Other definitions for saccharin that I've seen before include "Artificial sweetener" , "alternative sweetener" , "Very sweet compound" , "Synthetic sweetener" , "Sweet substance used as sugar substitute" .)