Explanatory tool (7)
I believe the answer is:
analogy
'explanatory tool' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for analogy that I've seen before include "Partial likeness between two things being compared" , "A comparison or parallel" , "Instructive comparison between two things" , "A clarifying comparison" , "Comparison made to aid clarification" .)