Old tax office work finally getting a little tedious (7)
I believe the answer is:
irksome
'tedious' is the definition.
(I know that tedious can be written as irksome)
'old tax office work finally getting a little' is the wordplay.
'old tax office' becomes 'IR' (Inland Revenue, predecessor of HMRC).
'finally' indicates one should take the final letters.
'getting' says to put letters next to each other.
'a little' becomes 'some' (I've seen this before).
The last letter of 'work' is 'k'.
'ir'+'k'+'some'='IRKSOME'
(Other definitions for irksome that I've seen before include "Burdensome and tedious" , "Tiresome, irritating" , "Troublesome, vexing" , "Annoying or tiresome" , "'Annoying, irritating or boring (7)'" .)