Old yard in South Africa producing plant (4)
I believe the answer is:
soya
'plant' is the definition.
(soya is a kind of plant)
'old yard in south africa' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'yard' becomes 'y' ().
'in' means one lot of letters goes inside another.
'south africa' becomes 'sa' (abbreviation).
'o'+'y'='oy'
'oy' put into 'sa' is 'SOYA'.
'producing' acts as a link.
(Other definitions for soya that I've seen before include "Oily bean" , "It's used as a substitute for animal protein" , "Type of bean" , "Legume" , "I'm rich in oil" .)