Rested on side, if abnormally happy (9)
I believe the answer is:
satisfied
'happy' is the definition.
(thesaurus)
'rested on side if abnormally' is the wordplay.
'rested' becomes 'sat' (I've seen this before).
'on' indicates putting letters inside.
'abnormally' indicates an anagram.
'side'+'if'='sideif'
'sideif' anagrammed gives 'isfied'.
'sat' put within 'isfied' is 'SATISFIED'.
(Other definitions for satisfied that I've seen before include "Content" , "as one may be with a second helping" , "Convinced" , "relieved of doubt" , "Well catered for" .)