Felix is happy in it (5)
I believe the answer is:
latin
'felix is happy in it' is the definition.
I can't tell whether this definition defines the answer.
'felix is happy in it' is the wordplay.
'felix' becomes 'la' (I can't justify 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" .)