The illegal substance, catch one putting into tins (8)
I believe the answer is:
cannabis
'the illegal substance' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'catch one putting into tins' is the wordplay.
I cannot really see how this works, but
'catch' could be 'nab' (nab can mean to capture or arrest) and 'nab' is found in the answer.
'one' could be 'an' (an apple is one apple) and 'an' is found within the answer.
The remaining letters 'cis' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cannabis that I've seen before include "Most commonly used (mostly) illicit drug" , "Strangely, Incas ban this drug" , "Pot" , "Smoked drug" , "Hemp plant, illegal intoxicant drug" .)