This writer's impatient expression characterised by mischief? (6)
I believe the answer is:
impish
'expression characterised by mischief?' is the definition.
I can't judge whether this defines the answer.
'this writer's impatient' is the wordplay.
'this writer's' becomes 'im' (the person writing the crossword clues).
'impatient' becomes 'pish' (I can't explain this - if you can you should give a lot more credence to this answer).
'im'+'pish'='IMPISH'
Can you help me to learn more?
(Other definitions for impish that I've seen before include "troublesome" , "Naughty" , "Mischievous like small devil" , "Wicked" , "Playful" .)