Impressed by the fancy crust on a piece of cake (6)
I believe the answer is:
struck
'impressed' is the definition.
('strike' can be a synonym of 'impress')
'the fancy crust on a piece of cake' is the wordplay.
'the fancy' is an anagram indicator.
'on' is a charade indicator (letters next to each other).
'a piece of cake' becomes 'k' (I am not sure about this - if you are sure you should believe this answer much more).
'crust' with letters rearranged gives 'struc'.
'struc'+'k'='STRUCK'
'by' acts as a link.
Can you help me to learn more?
(Other definitions for struck that I've seen before include "Hit as by lightning" , "(Of clock) chimed" , "Thumped" , "Took industrial action" , "Hit or impressed" .)