What one may do with a rubber in Denver? A secret! (5)
I believe the answer is:
erase
'what one may do with a rubber' is the definition.
(rubber can mean an eraser)
'in denver? a secret' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ERASE' is hidden within 'denver a secret'.
(Other definitions for erase that I've seen before include "Scratch" , "Kill" , "Remove evidence of" , "Blot out" , "Blank out" .)