Oxygen used in southwestern plant (3)
I believe the answer is:
sow
'plant' is the definition.
'oxygen used in southwestern' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'used in' indicates putting letters inside.
'southwestern' becomes 'sw'.
'o' placed inside 'sw' is 'SOW'.
(Other definitions for sow that I've seen before include "Broadcast" , "Female pig" , "Prepare farmland" , "Scatter seed; pig" , "Plant seed in soil" .)