21 down
Page
and
covers
–
those
to
check?
(6)
Page and covers – those to check? (6)
hints
0
checks
0
hints
The definition is at the end of the clue.
How the clue works
Look at those to check? — that's your definition, pointing at printer's proofs awaiting review. You get there by taking P (abbreviation for page) and adding ROOFS (covers, as in a roof covers a building).