Happy about relative being compatible (10)
I believe the answer is:
consistent
'compatible' is the definition.
(I've seen this before)
'happy about relative' is the wordplay.
'happy' becomes 'content' (associated in meaning).
'about' means one lot of letters goes inside another.
'relative' becomes 'sis' (short for sister).
'content' enclosing 'sis' is 'CONSISTENT'.
'being' acts as a link.
(Other definitions for consistent that I've seen before include "Unchanging in standard over time" , "true to priniciples" , "stable" , "the same as always" , "true to principles" .)