Pipes black within a day (5)
I believe the answer is:
tubes
'pipes' is the definition.
(I know this)
'black within a day' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil classifications).
'within' means one lot of letters goes inside another.
'a day' becomes 'tues' (abbreviation for Tuesday).
'b' put into 'tues' is 'TUBES'.
(Other definitions for tubes that I've seen before include "Hoses, pipes, lines etc" , "Piping" , "Hollow cylindrical shapes" , "Long hollow cylinders" , "Long hollow conduits" .)