Contemptuously replacing nitrogen with chlorine at regular intervals (10)
I believe the answer is:
'at regular intervals' is the definition.
'contemptuously replacing nitrogen with chlorine' is the wordplay.
I cannot quite understand how this works, but
'chlorine' could be 'cl' (chemical symbol) and 'cl' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?