Run if snake is found under loo’s lid (6)
I believe the answer is:
ladder
'run' is the definition.
(both can mean a rip in tights)
'snake is found under loo's lid' is the wordplay.
'snake' becomes 'adder' (adder is a kind of snake).
'is found under' means one lot of letters go next to another.
'lid' says to take the initial letters (the top, or first, letter).
The first letter of 'loos' is 'l'.
'adder' put after 'l' is 'LADDER'.
'if' is the link.
(Other definitions for ladder that I've seen before include "Companionway" , "ranking table" , "Domestic device for climbing" , "Set of rungs" , "snag in hose" .)