Fruit spread on the turn, if not rank (5)
I believe the answer is:
major
'rank' is the definition.
(military rank)
'fruit spread on the turn if not' is the wordplay.
'fruit spread' becomes 'jam' (I can't justify this - if you can you should give a lot more credence to this answer).
'on the turn' says the letters should be written backwards.
'if not' becomes 'or' (both can be said before introducing an alternative).
'jam' back-to-front is 'maj'.
'maj'+'or'='MAJOR'
Can you help me to learn more?
(Other definitions for major that I've seen before include "Very significant - army officer" , "Significant; rank" , "Pre-eminent" , "John --, ex-PM" , "Conservative PM" .)