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 one could define the other.
'the niece to be possibly about five' is the wordplay.
'to be possibly' is an anagram indicator.
'about' is an insertion indicator.
'five' becomes 'V' (Roman numeral).
'niece' is an anagram of 'eince'.
'eince' going around 'v' is 'EVINCE'.
'show' is the 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" .)