Not the one to upset Rosemary, perhaps, in short (5)
I believe the answer is:
other
'not the one' is the definition.
(ie another one)
'to upset rosemary perhaps in short' is the wordplay.
'upset' shows that the letters should be reversed in order.
'rosemary perhaps' becomes 'herb' (rosemary is a kind of herb).
'in short' means to remove the last letter.
'to' in reverse letter order is 'ot'.
'herb' with its final letter taken off is 'her'.
'ot'+'her'='OTHER'
(Other definitions for other that I've seen before include "Different, not the same one or this one" , "Different; additional" , "Second" , "Not the one mentioned" , "One may be significant" .)