Seen as a cake without topping (7)
I believe the answer is:
noticed
'seen' is the definition.
(I've seen this before)
'a cake without topping' is the wordplay.
I cannot really 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 located in the answer.
'without' could be 'no' and 'no' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'as' acts as a link.
(Other definitions for noticed that I've seen before include "Became aware of" , "Detected" , "remarked" , "Observed or saw" , "Seen" .)