Would show the niece to be possibly about five (6)
I believe the answer is:
evince
'would' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how they can define each other.
'the niece to be possibly about five' is the wordplay.
'to be possibly' indicates anagramming the letters.
'about' is an insertion indicator.
'five' becomes 'V' (Roman numeral).
'niece' with letters rearranged gives 'eince'.
'eince' enclosing 'v' is 'EVINCE'.
'show' acts as a link.
Can you help me to learn more?
(Other definitions for evince that I've seen before include "Express,show" , "Express, show" , "Prove beyond doubt" , "Venice (anag.)" , "Make evident" .)