One cracking grin, perhaps as happy as Larry (6)
I believe the answer is:
simile
'one cracking grin perhaps as happy as larry' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'one cracking grin perhaps' is the wordplay.
'one' becomes 'i' (Roman numeral).
'cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'grin perhaps' becomes 'smile' (grinning is a kind of smiling).
'i' put into 'smile' is 'SIMILE'.
Can you help me to learn more?
(Other definitions for simile that I've seen before include "Figure of speech that expresses a resemblance between different things" , "Lies I'm finding in figure of speech" , "Figure of speech making straightforward comparison" , "Figure of speech, eg, as cool as a cucumber" , "one being as good as gold" .)