Left in agonies, sickening for a piece of cake (5,7)
I believe the answer is:
plain sailing
'a piece of cake' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'left in agonies sickening' is the wordplay.
'left' becomes 'l' (common abbreviation).
'in' means one lot of letters goes inside another.
'agonies' becomes 'pains'.
'sickening' becomes 'ailing' (I am not sure about this - if you are sure you should believe this answer much more).
'l' inserted inside 'pains' is 'plains'.
'plains'+'ailing'='PLAIN-SAILING'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for plain sailing that I've seen before include "Easy and unobstructed progress" , "A breeze" , "No problem (for yachtsman?)" , "Trouble-free progress" , "Progress without obstruction" .)