Serving no purpose (6)
'serving no purpose' is the definition.
(having no purpose or function)
This is the entire clue.
(Other definitions for useless that I've seen before include "to no avail" , "Vain - unavailing" , "Is Sue less impractical?" , "Unserviceable" , "Good-for-nothing" .)
'serving no purpose' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for otiose that I've seen before include "Serving no good purpose" , "O so tie up and be vain and empty" , "unnecessary to say" , "Idle, empty" , "Indolent, lazy" .)