Happy to hold relative steady (10)
I believe the answer is:
consistent
'steady' is the definition.
(thesaurus)
'happy to hold relative' is the wordplay.
'happy' becomes 'content' ('content' is associated in meaning with 'happy'**).
'to hold' means one lot of letters goes inside another.
'relative' becomes 'sis' (short for sister).
'content' placed around 'sis' is 'CONSISTENT'.
(Other definitions for consistent that I've seen before include "true to priniciples" , "true to principles" , "Steady" , "stable" , "the same as always" .)