Stocking only second-rate nitrogen (5)
I believe the answer is:
nylon
'stocking' is the definition.
(nylon stocking is a kind of stocking)
'only second-rate nitrogen' is the wordplay.
I cannot really understand how this works, but
an anagram of 'only' is 'nylo' which is located in the answer.
'nitrogen' could be 'n' (N is the chemical symbol for nitrogen) and 'n' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nylon that I've seen before include "Fibre of polymeric amides" , "The first synthetic fibre" , "Synthetic polymer" , "Artificial fibre" , "where ladder can appear?" .)