Large snake? Run! (6)
I believe the answer is:
ladder
'run' is the definition.
(both can mean a rip in tights)
'large snake' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'snake' becomes 'adder' (type of snake).
'l'+'adder'='LADDER'
(Other definitions for ladder that I've seen before include "something that can send you in the opposite direction?" , "A run in a stocking" , "sometimes with companion" , "Hierarchical structure" , "Support for climber" .)