Like a plain cake that's observed? (7)
I believe the answer is:
noticed
'observed?' is the definition.
('noticed' can be similar in meaning to 'observed')
'like a plain cake' 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 located in the answer.
This explanation may well be incorrect...
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for noticed that I've seen before include "Realised" , "Perceived or observed" , "Spotted" , "Became aware of" , "Marked" .)