Day in Coleraine (3)
I believe the answer is:
era
'day' is the definition.
(I've seen this in another clue)
'in coleraine' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ERA' is hidden within 'coleraine'.
(Other definitions for era that I've seen before include "Aeon" , "Long historic period" , "Significant time" , "Stage" , "Special time" .)