Understanding length in general, initially (9)
I believe the answer is:
fathoming
'understanding' is the definition.
(fathoming is a kind of understanding)
'length in general initially' is the wordplay.
I cannot really understand how this works, but
'in' is within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fathoming that I've seen before include "Finally seeing" , "Getting" , "Grasping" .)