Plain cake small once cooked (5)
I believe the answer is:
scone
'plain cake' is the definition.
(I've seen this before)
'small once cooked' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'cooked' is an anagram indicator.
'once' with letters rearranged gives 'cone'.
's'+'cone'='SCONE'
(Other definitions for scone that I've seen before include "Griddle-cake" , "Perth's settlement" , "Small plain cake and Scottish town" , "tea time treat" , "Tea-time treat" .)