Irksome boy's back in Hertfordshire town (6)
I believe the answer is:
trying
'irksome' is the definition.
(thesaurus)
'boy's back in hertfordshire town' is the wordplay.
'back' suggests the final letters.
'in' means one lot of letters goes inside another.
'hertfordshire town' becomes 'tring' (I've seen this before).
The final letter of 'boy' is 'y'.
'y' put inside 'tring' is 'TRYING'.
(Other definitions for trying that I've seen before include "Working in a court can be" , "Tiresome" , "Upsetting" , "work on bench" , "Putting into dock" .)