Large snake -- not something one wants to find in one's tights (6)
I believe the answer is:
ladder
'not something one wants to find in one's tights' is the definition.
(ladder can mean a tear in a pair of tights)
'large snake' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'snake' becomes 'adder' (adder is a kind of snake).
'l'+'adder'='LADDER'
(Other definitions for ladder that I've seen before include "Support for climber" , "Run in stockings" , "Means of ascent" , "Row of unravelled stitches" , "Vertical run in tights" .)