Moose knocks over beam (7)
I believe the answer is:
sparkle
'beam' is the definition.
(thesaurus)
'moose knocks over' is the wordplay.
'moose' becomes 'elk' (synonyms).
'knocks' becomes 'raps' ('rap' can be a synonym of 'knock').
'over' says the letters should be written backwards.
'elk'+'raps'='elkraps'
'elkraps' back-to-front is 'SPARKLE'.
(Other definitions for sparkle that I've seen before include "exhibit vivacity" , "showing wit" , "Shine (wittily)" , "Shine with reflected light" , "Glittering light" .)