Felix is happy in it (5)

I believe the answer is:

latin

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'felix is happy in it' is the definition.
I can't judge whether this defines the answer.

'felix is happy in it' is the wordplay.
'felix' becomes 'la' (I can't explain this - if you can you should believe this answer much more).
'is happy' is an anagram indicator (happy can mean slightly drunk).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'in'+'t'='int'
'int' with letters rearranged gives 'tin'.
'la'+'tin'='LATIN'

Can you help me to learn more?

(Other definitions for latin that I've seen before include "Language of old Romans" , "Parent language of French,Romanian" , "Language of Cicero" , "Language of old 8 down" , "The language of Virgil" .)

I've seen this clue in the Evening Standard.

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