Twig, a unit of distance (6)
I believe the answer is:
fathom
'twig' is the definition.
(thesaurus)
'a unit of distance' is the wordplay.
I cannot quite understand how this works, but
'a' is present in the answer.
'unit' could be 't' (T is an example) and 't' is located in the answer.
'of' could be 'o' and 'o' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for fathom that I've seen before include "Come to understand - unit of depth" , "Comprehend - a depth of six feet" , "Measure of depth, 6 ft." , "Come to understand the water depth" , "Measure of water depth" .)