Even short relative enters happy (10)
I believe the answer is:
consistent
'even' is the definition.
(thesaurus)
'short relative enters happy' is the wordplay.
'short relative' becomes 'sis' (short for sister; 'short' asks for a short form).
'enters' is an insertion indicator.
'happy' becomes 'content' (associated in meaning).
'sis' going inside 'content' is 'CONSISTENT'.
(Other definitions for consistent that I've seen before include "true to priniciples" , "stable" , "Unchanging in standard over time" , "true to principles" , "uniform" .)