Shape on the page not entirely irregular (8)
I believe the answer is:
heptagon
'shape' is the definition.
(heptagon is a kind of polygon)
'on the page not entirely irregular' is the wordplay.
'not entirely' means to remove the last letter (not all of the word).
'irregular' indicates an anagram.
'on'+'the'+'page'='onthepage'
'onthepage' with its last letter taken off is 'onthepag'.
'onthepag' is an anagram of 'HEPTAGON'.
(Other definitions for heptagon that I've seen before include "A geometric figure with seven sides" , "Seven-sided shape" , "Seven-sided object" , "Seven-sided figure" .)