It follows like one breaking into joyful expression (6)
I believe the answer is:
simile
'it follows' is the definition.
The definition suggests a singular noun which matches the answer.
'one breaking into joyful expression' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
'expression' could be 'smile' (I have seen 'Happy expression' mean 'smile' so perhaps 'expression' could also mean 'smile') and 'smile' is found within the leftover letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'like' is the link.
Can you help me to learn more?
(Other definitions for simile that I've seen before include "The likening of one thing to another" , "Figure of speech, eg, as cool as a cucumber" , "Figure of speech involving comparison" , "Figure of speech making simple comparison" , "Figure of speech expressing resemblance between different things" .)