School tests content of sumac fruit (7)
I believe the answer is:
satsuma
'fruit' is the definition.
(satsuma is a kind of fruit)
'school tests content of sumac' is the wordplay.
'school tests' becomes 'sats' (scholastic assessment tests in the UK).
'content of' indicates the central letters.
The central letters of 'sumac' are 'uma'.
'sats'+'uma'='SATSUMA'
(Other definitions for satsuma that I've seen before include "Small, easily-peeled orange" , "Tangerine-like fruit" , "Segmented item" , "Japanese province and small orange" , "Orange, district and style of Japanese pottery" .)