Understand it to be a figure (5)
I believe the answer is:
digit
'a figure' is the definition.
(thesaurus)
'understand it' is the wordplay.
'understand' becomes 'dig' (digging is a kind of understanding).
'dig'+'it'='DIGIT'
'to be' acts as a link.
(Other definitions for digit that I've seen before include "One in ten, say" , "Finger; number" , "one's given?" , "a number less than ten" , "using 0 or 1 for example" .)