Chapter on law imposed on American plant (6)
I believe the answer is:
cactus
'plant' is the definition.
(cactus is a kind of plant)
'chapter on law imposed on american' is the wordplay.
'chapter' becomes 'c'.
'on' says to put letters next to each other.
'law' becomes 'act' (I've seen this before).
'imposed on' is a charade indicator (letters next to each other).
'american' becomes 'us'.
'c'+'act'+'us'='CACTUS'
(Other definitions for cactus that I've seen before include "Drought-resistant plant" , "Prickly character" , "Spiny, succulent plant" , "Desert flower" , "Spiny desert plant" .)