Collection artist mounted on beam (5)
I believe the answer is:
array
'collection' is the definition.
(I've seen this in another clue)
'artist mounted on beam' is the wordplay.
'artist' becomes 'RA' (Royal Academician, member of Royal Academy of Arts).
'mounted' says the letters should be written backwards (in a down clue, letters mounted up).
'on' says to put letters next to each other.
'beam' becomes 'ray' (synonyms).
'ra' back-to-front is 'ar'.
'ar'+'ray'='ARRAY'
(Other definitions for array that I've seen before include "Impressive display or range of something" , "Significant arrangement" , "dress" , "Impressive exhibition" , "Orderly or impressive display" .)