Doctor ate a little cookie? OK, a biscuit! (7)
I believe the answer is:
oatcake
'biscuit' is the definition.
(I've seen this before)
'doctor ate a little cookie? ok a' is the wordplay.
'doctor' is an anagram indicator.
'a little' indicates taking the first letters.
The initial letter of 'cookie' is 'c'.
'ate'+'c'+'ok'+'a'='atecoka'
'atecoka' with letters rearranged gives 'OATCAKE'.
(Other definitions for oatcake that I've seen before include "something like a biscuit" , "Dry biscuit" , "Traditional Scots biscuit" , "Savoury biscuit" , "Variety of biscuit" .)