School filled with sound of cow pet (6)
I believe the answer is:
smooch
'pet' is the definition.
(both can mean a romantic embrace)
'school filled with sound of cow' is the wordplay.
'school' becomes 'sch'.
'filled with' means one lot of letters goes inside another.
'sound of cow' becomes 'moo' (I've seen this before).
'sch' enclosing 'moo' is 'SMOOCH'.
(Other definitions for smooch that I've seen before include "neck" , "Kiss and caress slangily" , "Dance slowly while in an embrace - kiss - pet" , "Canoodle" , "Kiss and cuddle (colloq.)" .)