Start of long summer run (6)
I believe the answer is:
ladder
'run' is the definition.
(both can mean a rip in tights)
'start of long summer' is the wordplay.
'start of' says to take the initial letters.
'summer' becomes 'adder' (someone who adds or does sums).
The first letter of 'long' is 'l'.
'l'+'adder'='LADDER'
(Other definitions for ladder that I've seen before include "Set of steps" , "Opposite of snake?" , "Climbing steps or rungs" , "Household climbing device or tear in stocking" , "Framework for scaling, usually portable" .)