Fifth of puzzles by Serpent? Run! (6)
I believe the answer is:
ladder
'run' is the definition.
(both can mean a rip in tights)
'fifth of puzzles by serpent?' is the wordplay.
'fifth of puzzles' becomes 'l' (5th letter of 'puzzles').
'by' says to put letters next to each other.
'serpent?' becomes 'adder' (type of snake).
'l'+'adder'='LADDER'
(Other definitions for ladder that I've seen before include "Climbing steps or rungs" , "Household climbing device or hole in stocking" , "Unravel (as stocking)" , "A fire-fighter may use one" , "sometimes with companion" .)