Before the end of the day, the trial will make you irritable (5)
I believe the answer is:
testy
'irritable' is the definition.
(testy can mean irritable or annoyed)
'before the end of the day the trial' is the wordplay.
'before' says to put letters next to each other.
'the end of the' indicates one should take the final letters.
'the trial' becomes 'test' ('test' can be a synonym of 'trial').
The last letter of 'day' is 'y'.
'y' put after 'test' is 'TESTY'.
'will make you' is the link.
(Other definitions for testy that I've seen before include "Irascible" , "Cross" , "bad temper" , "likely to flare up?" , "Bad-tempered" .)