A cub - it is playing with a fawn (7)
I believe the answer is:
biscuit
'a fawn' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'a cub it is playing' is the wordplay.
'it is' becomes 'itis'.
'playing' indicates anagramming the letters.
'cub'+'itis'='cubitis'
'cubitis' is an anagram of 'BISCUIT'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for biscuit that I've seen before include "Eg, ginger nut" , "Unglazed pottery" , "Eg, custard cream" , "Baked item; light brown" , "Custard cream, perhaps" .)