Saw what plain cake was? (7)
I believe the answer is:
noticed
'saw' is the definition.
(I've seen this before)
'plain cake was?' is the wordplay.
I cannot quite see how this works, but
'cake' could be 'ice' (I have seen 'Top cake' mean 'ice' so perhaps 'cake' could also mean 'ice') and 'ice' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
'what' is the link.
Can you help me to learn more?
(Other definitions for noticed that I've seen before include "Observed or saw" , "Spotted" , "remarked" , "Detected" , "Realised" .)