Rung here a number of times (6)
I believe the answer is:
'rung here a' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'number of times' is the wordplay.
'number' becomes 'L' (L is an example).
'of times' becomes 'adder' (I can't explain this - if you can you should give a lot more credence to this answer).
Can you help me to learn more?
(Other definitions for ladder that I've seen before include "Unravel (as stocking)" , "A fire-fighter may use one" , "ranking table" , "Household climbing device or hole in stocking" , "Vertical run in tights" .)