Chemical substance picked up in nocturnal journey (7)
I believe the answer is:
nitride
'chemical substance' is the definition.
(I know that nitride is a type of chemical substance)
'picked up in nocturnal journey' is the wordplay.
'picked up in' shows that the letters should be reversed in order (in a down clue, letters go up).
'nocturnal' becomes 'tin' (I am not sure about this - if you are sure you should believe this answer much more).
'journey' becomes 'ride' (ride is a kind of journey).
'tin' back-to-front is 'nit'.
'nit'+'ride'='NITRIDE'
Can you help me to learn more?
(Another definition for nitride that I've seen is " Compound".)