Observed how plain such a cake is (7)
I believe the answer is:
noticed
'observed' is the definition.
(noticing is a kind of observing)
'plain such a cake is' is the wordplay.
I cannot quite understand 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 within the answer.
This may be the basis of the clue (or it may be nonsense).
'how' is the link.
Can you help me to learn more?
(Other definitions for noticed that I've seen before include "Detected" , "Became aware of" , "Realised" , "regarded" , "Marked" .)