Heavy demand for a ladder (3)
I believe the answer is:
run
This clue is a double definition.
'heavy demand' is the first definition.
(I've seen this in another clue)
'ladder' is the second definition.
(both can mean a tear in tights)
'for a' acts as a link.
(Other definitions for run that I've seen before include "Enter a race" , "Dart" , "journey" , "Unit of score in cricket" , "string of performances" .)