Sound like a cow in school, but get kissing (6)
I believe the answer is:
smooch
'kissing' is the definition.
(smooch means to kiss)
'sound like a cow in school' is the wordplay.
'sound like a cow' becomes 'moo' (a cow makes a moo sound).
'in' is an insertion indicator.
'school' becomes 'sch' (abbreviation).
'moo' inserted into 'sch' is 'SMOOCH'.
'but get' acts as a link.
(Other definitions for smooch that I've seen before include "Snuggle" , "Canoodle" , "Kiss and caress slangily" , "neck" , "Kiss and cuddle (colloq.)" .)