Annoyance about working hard finally decreased (6)
I believe the answer is:
ironed
'decreased' is the definition.
(I've seen this in another clue)
'annoyance about working hard finally' is the wordplay.
'annoyance' becomes 'ire' (I've seen this before).
'about' indicates putting letters inside.
'working' becomes 'on' (on can mean working or activated).
'finally' suggests the final letters.
The final letter of 'hard' is 'd'.
'ire' enclosing 'on' is 'irone'.
'irone'+'d'='IRONED'
(Other definitions for ironed that I've seen before include "Smoothed cloth with flatbased implement" , "Made smooth, shirt or collar" , "flattened" , "Removed creases" , "Smoothed the cloth with heat" .)