It grows in the south of Iran (3)
I believe the answer is:
fir
'it grows' is the definition.
The definition suggests a singular noun which matches the answer.
'in the south of iran' is the wordplay.
'in the south' says to take the final letters (I've seen 'in the south of' mean this (in a down clue, the bottom letter)).
'iran' becomes 'ir' (eg in website addresses).
The last letter of 'of' is 'f'.
'f'+'ir'='FIR'
Can you help me to learn more?
(Other definitions for fir that I've seen before include "Evergreen tree for Irish men?" , "Scots pine" , "Coniferous tree" , "wood" , "Tree for Irishmen" .)