Pipe a short time blocked by stone (7)
I believe the answer is:
whistle
'pipe' is the definition.
(I know that whistle is a type of fipple flute)
'a short time blocked by stone' is the wordplay.
'a short time' becomes 'while' (while is a kind of short time).
'blocked by' indicates putting letters inside (upper walls off glass inside formal).
'stone' becomes 'st' (abbreviation).
'while' enclosing 'st' is 'WHISTLE'.
(Other definitions for whistle that I've seen before include "Become an informer" , "something needing wetting?" , "He wilts (anag.)" , "Call vainly for" , "Referee's item" .)