Test a southern state (5)
I believe the answer is:
assay
'test' is the definition.
(I know that assay is a type of test)
'a southern state' is the wordplay.
'southern' becomes 's' (abbreviation).
'state' becomes 'say' (speak).
'a'+'s'+'say'='ASSAY'
(Other definitions for assay that I've seen before include "Test ore for value" , "Test for metal content" , "Examine chemicals" , "'Analyse, precious metal, say (5)'" , "Test of substance" .)