Old US president sent back the French unit of wheat (6)
I believe the answer is:
bushel
'unit of wheat' is the definition.
'bushel' can be an answer for 'unit' (bushel is a kind of unit). I'm not sure about the 'of wheat' bit.
'old us president sent back the french' is the wordplay.
'old us president' becomes 'bush' (I can't explain this - if you can you should believe this answer much more).
'sent back' says the letters should be written backwards.
'the french' becomes 'le' ('the' in French).
'le' in reverse letter order is 'el'.
'bush'+'el'='BUSHEL'
Can you help me to learn more?
(Other definitions for bushel that I've seen before include "Eight-gallon measure of capacity - under which one's light should not be hidden" , "Dry measure of eight gallons" , "Measure equalling eight Imperial gallons" , "A few pecks wanted" , "Old measure of capacity" .)