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' suggests 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 final 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" .)