Steady relative in happy surroundings (10)
I believe the answer is:
consistent
'steady' is the definition.
(thesaurus)
'relative in happy surroundings' is the wordplay.
'relative' becomes 'sis' (short for sister).
'in happy' becomes 'content' ('content' is associated in meaning with 'happy'. I am not sure about the 'in' bit.**).
'surroundings' indicates putting letters inside.
'sis' inserted within 'content' is 'CONSISTENT'.
(Other definitions for consistent that I've seen before include "Steady" , "the same as always" , "in agreement" , "uniform" , "Unchanging in standard over time" .)