Cake is solid on top once cooked (5)
I believe the answer is:
scone
'cake' is the definition.
(scone is a type of cake)
'solid on top once cooked' is the wordplay.
'on top' suggests taking the first letters.
'cooked' indicates an anagram (letters cooked into a new form).
The initial letter of 'solid' is 's'.
'once' anagrammed gives 'cone'.
's'+'cone'='SCONE'
'is' acts as a link.
(Other definitions for scone that I've seen before include "ancient Scottish settlement" , "baked cake" , "Cream tea cake" , "tea time treat" , "Unsweetened cake" .)