Seriously small beast adopting rejected stray (3,4)
I believe the answer is:
for real
'seriously' is the definition.
(thesaurus)
'small beast adopting rejected stray' is the wordplay.
'small beast' becomes 'foal'.
'adopting' means one lot of letters goes inside another.
'rejected' says the letters should be written backwards.
'stray' becomes 'err' ('err' can be a synonym of 'stray').
'err' in reverse letter order is 'rre'.
'foal' enclosing 'rre' is 'FOR REAL'.
(Other definitions for for real that I've seen before include "certainly" , "Serious" .)