Will fetch, you understand (7)
I believe the answer is:
'understand' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for realise that I've seen before include "Grasp", "Get the idea?", "Achieve, make", "catch on", "Come to understand - see liar (anag)".)