Diameter on shower tap (5)
I believe the answer is:
drain
'tap' is the definition.
(thesaurus)
'diameter on shower' is the wordplay.
'diameter' becomes 'd' (mathematics).
'on' says to put letters next to each other (in a down clue, letters appear on others).
'shower' becomes 'rain' (rainfall).
'd'+'rain'='DRAIN'
(Other definitions for drain that I've seen before include "Draw off U sewer" , "Something using up resources" , "Remove liquid from" , "Waste pipe" , "See 11 dn" .)