Stressful day's ending in Hertfordshire town (6)
I believe the answer is:
trying
'stressful' is the definition.
('trying' can be a synonym of 'stressful')
'day's ending in hertfordshire town' is the wordplay.
'ending' 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 'day' is 'y'.
'y' inserted within 'tring' is 'TRYING'.
(Other definitions for trying that I've seen before include "Sitting in judgement" , "Testing" , "Attempting or irritating" , "Attempting; difficult" , "job for a jury?" .)