Understood it's filed under 'G' (7)
I believe the answer is:
grasped
'understood' is the definition.
(grasping is a kind of understanding)
'filed under g' is the wordplay.
I cannot quite see how this works, but
'g' is within the answer.
The remaining letters 'rasped' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
'it's' is the link.
Can you help me to learn more?
(Other definitions for grasped that I've seen before include "Held firmly or got the meaning" , "Understood" , "Got the picture" , "Took hold of or got the meaning" .)