Promote happy class (7)
I believe the answer is:
upgrade
'promote' is the definition.
(I know that promote can be written as upgrade)
'happy class' is the wordplay.
'happy' becomes 'up' (I've seen this in other clues).
'class' becomes 'grade' (grading is a kind of classing).
'up'+'grade'='UPGRADE'
(Other definitions for upgrade that I've seen before include "Higher-standard version" , "Drug ape to improve" , "Raise in status or quality" , "Improve, enhance" , "Raise in value or esteem" .)