3 down
Poorly
writer’s
in
favour
of
vague
reply
(10)
Poorly writer’s in favour of vague reply (10)
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The definition is at the beginning of the clue.
How the clue works
Poorly is the definition, meaning "in an improper manner". The answer is built in three parts: I'M (as in "this writer is", giving "writer's") followed by PRO (meaning "in favour of"), then an anagram of REPLY indicated by "vague", which rearranges to PERLY. Put them all together and you get IMPROPERLY.