A very small measure of corn (5)
I believe the answer is:
grain
This clue could be a double definition.
'a very small measure' is the first definition.
'grain' can be an answer for 'measure' (grain is a kind of measure). I'm unsure of the 'a very small' bit.
'corn' is the second definition.
(I've seen this before)
'of' acts as a link.
Can you help me to learn more?
(Other definitions for grain that I've seen before include "Small quantity (of corn etc)" , "Eg, wheat" , "Cereal as food" , "Tiny amount" , "Character" .)