Measure round size of paper (6)
I believe the answer is:
quarto
'size of paper' is the definition.
(I've seen this before)
'measure round' is the wordplay.
'measure' becomes 'quart' (quart is a kind of measure**).
'round' becomes 'o' (round can mean a round object).
'quart'+'o'='QUARTO'
(Other definitions for quarto that I've seen before include "Publishing term, four leaves are formed from one sheet" , "Ten-by-eight inch paper" , "Book size resulting in folding one sheet four times" , "Book page size" , "Twice-folded paper size" .)