Understand imperial measure (6)
I believe the answer is:
fathom
This clue could be a double definition.
'understand' is the first definition.
(to fathom something is to understand it)
'imperial measure' is the second definition.
'fathom' can be an answer for 'measure' (fathom is a kind of measure). I'm not sure about the 'imperial' bit.
Can you help me to learn more?
(Other definitions for fathom that I've seen before include "Unit of depth - comprehend it" , "Discover the meaning of - six feet" , "Come to understand - unit of depth" , "Unit of six feet in measuring depth of water" , "Nautical measure of depth" .)