Snake seen behind opening in lavatory, run! (6)
I believe the answer is:
ladder
'run' is the definition.
(both can mean a rip in tights)
'snake seen behind opening in lavatory' is the wordplay.
'snake' becomes 'adder' (adder is a kind of snake).
'seen behind' says to put letters next to each other (some letters go behind others).
'opening in' indicates taking the first letters.
The first letter of 'lavatory' is 'l'.
'adder' after 'l' is 'LADDER'.
(Other definitions for ladder that I've seen before include "something that can send you in the opposite direction?" , "Hierarchical structure" , "Unravel (as stocking)" , "sort of tournament" , "Means of ascent" .)