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 cannot see how they can define each other.
'one cracking grin perhaps' is the wordplay.
'one' becomes 'i' (Roman numeral).
'cracking' means one lot of letters goes inside another (inserted letters go into a gap or crack).
'grin perhaps' becomes 'smile' (grinning is a kind of smiling).
'i' inserted within '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" .)