School tests, I'm thinking, are in Mandarin (7)
I believe the answer is:
satsuma
'mandarin' is the definition.
(I know that satsuma is a type of mandarin)
'school tests i'm thinking are' is the wordplay.
'school tests' becomes 'SATs' (scholastic assessment tests in the UK).
'i'm thinking' becomes 'um' (I can't justify this - if you can you should believe this answer much more).
'are' becomes 'a' (short for 'are', historical unit of measurement).
'sats'+'um'+'a'='SATSUMA'
'in' is the link.
Can you help me to learn more?
(Other definitions for satsuma that I've seen before include "Citrus fruit from Japan" , "Easy-peeling tangerine" , "Small, easily-peeled orange" , "A small citrus fruit" , "Citrus tree named after an old Japanese province" .)