Cycle courier entering health club to deliver plastering material (6)
I believe the answer is:
gypsum
'plastering material' is the definition.
(material used in plastering)
I don't understand the rest of the clue.
(Other definitions for gypsum that I've seen before include "Mineral used to make plaster of Paris" , "Ingredient of fertilizer" , "Base of plaster of Paris" , "Alabaster" , "Spy mug made from this white mineral" .)