Seen as plain cake? (7)
I believe the answer is:
noticed
'seen' is the definition.
(I've seen this before)
'plain cake?' 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 found within the answer.
This explanation may well be incorrect...
'as' is the link.
(Other definitions for noticed that I've seen before include "Marked" , "Detected" , "remarked" , "Seen" , "Clocked" .)