Difficult day's ending in Hertfordshire town (6)
I believe the answer is:
trying
'difficult' is the definition.
(similar in meaning)
'day's ending in hertfordshire town' is the wordplay.
'ending' suggests the final letters (the ending of the word).
'in' means one lot of letters goes inside another.
'hertfordshire town' becomes 'tring' (town in Hertfordshire).
The final letter of 'day' is 'y'.
'y' going within 'tring' is 'TRYING'.
(Other definitions for trying that I've seen before include "Difficult - annoying" , "Testing" , "irksome" , "Making an effort - it's irritating" , "What could be witnessed in court" .)