Sent up Arab leader's old poem (4)
I believe the answer is:
rime
'old poem' is the definition.
(archaic spelling of 'rhyme')
'sent up arab leader's' is the wordplay.
'sent up' says the letters should be written backwards.
'arab leader' becomes 'emir' (I've seen this in another clue).
'emir' written backwards gives 'RIME'.
(Other definitions for rime that I've seen before include "Hoar frost" , "''The ... of the Ancient Mariner'' (Coleridge)" , "Ancient Mariner produced?" , "Frost left after freezing fog, or frozen dew" , "Ice" .)