Understanding old giant breaks horse (7)
I believe the answer is:
getting
'understanding' is the definition.
(getting is a kind of understanding)
'old giant breaks horse' is the wordplay.
'old giant' becomes 'ettin' (giant in Norse myth. I am not sure about the 'old' bit.).
'breaks' indicates putting letters inside (inserted letters break into the word).
'horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'ettin' going into 'gg' is 'GETTING'.
Can you help me to learn more?
(Other definitions for getting that I've seen before include "Receiving" , "Acquiring" , "Obtaining; becoming" .)