Random sides of sunlit beam (5)
I believe the answer is:
stray
'random' is the definition.
(I've seen this in another clue)
'sides of sunlit beam' is the wordplay.
'sides of' suggests removing the centre (only the letters on the sides remain).
'beam' becomes 'ray' (synonyms).
'sunlit' with its middle taken out is 'st'.
'st'+'ray'='STRAY'
(Other definitions for stray that I've seen before include "digress" , "Drift - isolated" , "Go off course" , "out of place" , "Wander off course" .)