Sing in Grenoble, do you, when climbing? (5)
I believe the answer is:
yodel
'sing' is the definition.
(I know that yodel is a more specific form of the action sing)
'in grenoble do you when climbing?' is the wordplay.
'in' indicates a hidden word.
'when climbing?' is a reversal indicator (in a down clue, letters go up).
'ledoy' is hidden within 'grenoble do you'.
'ledoy' in reverse letter order is 'YODEL'.
(Other definitions for yodel that I've seen before include "Alpine warble" , "Sing (Alpine folk music)" , "Alpine song or call" , "Sing (in the Alps)" , "Sing in traditional Swiss manner" .)