Smarten up stable hand (5)
I believe the answer is:
groom
I believe this is a double definition.
'smarten up' is the first definition.
(as in grooming oneself)
'stable hand' is the second definition.
(I've seen this before)
(Other definitions for groom that I've seen before include "royal attendant" , "Attendant for horses" , "Smarten" , "One caring for horses" , "new union member" .)