Busy retailers rarely see one when they open (4,5)
I believe the answer is:
late riser
'rarely see one when they open' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'busy retailers' is the wordplay.
'busy' indicates an anagram.
'retailers' with letters rearranged gives 'LATE RISER'.
(Other definitions for late riser that I've seen before include "Lie-abed" , "she spends half the day in bed" .)