Grain not left in square (4)
I believe the answer is:
four
'square' is the definition.
(four is a square number)
'grain not left' is the wordplay.
'grain' becomes 'fou' (I can't explain this - if you can you should give a lot more credence to this answer).
'not left' becomes 'r' (abbreviation for right).
'fou'+'r'='FOUR'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for four that I've seen before include "Set of bridge players?" , "An even number" , "Two squared" , "Boundary hit (cricket)" , "IV in Roman numerals" .)