Happy Beam is in action? (9)
I believe the answer is:
delighted
'happy' is the definition.
(I've seen this before)
'beam is in action?' is the wordplay.
'beam' becomes 'light' (synonyms).
'is in' indicates putting letters inside.
'action?' becomes 'deed' (deed is a kind of action).
'light' inserted into 'deed' is 'DELIGHTED'.
(Other definitions for delighted that I've seen before include "Over the moon" , "Joyful" , "Pleased as Punch" , "Greatly pleased" , "Overjoyed" .)