Day-long channel (5)
I believe the answer is:
ditch
'channel' is the definition.
(thesaurus)
'day-long' is the wordplay.
'day' becomes 'd'.
'long' becomes 'itch' (I've seen this before).
'd'+'itch'='DITCH'
(Other definitions for ditch that I've seen before include "Get rid of; dug channel" , "Abandon; one dug" , "Moat" , "give up" , "Narrow channel" .)