To the writer this is of absorbing significance (8,5)
I believe the answer is:
blotting paper
'to the writer this is' is the definition.
The definition suggests a singular noun which matches the answer.
'absorbing significance' is the wordplay.
'absorbing' becomes 'blotting' (blotting is a kind of absorbing).
'significance' becomes 'paper' (I can't explain this - if you can you should give a lot more credence to this answer).
'blotting'+'paper'='BLOTTING-PAPER'
'of' is the link.
Can you help me to learn more?
(Other definitions for blotting paper that I've seen before include "Ink soaker-upper" , "It absorbs ink" , "Absorber of ink" , "Ink-drying material" , "Ink drier" .)