Restrict movement of nitrates (8)
I believe the answer is:
straiten
'restrict' is the definition.
(I've seen this before)
'movement of nitrates' is the wordplay.
'movement of' indicates anagramming the letters (I've seen 'movement' mean this).
'nitrates' anagrammed gives 'STRAITEN'.
(Other definitions for straiten that I've seen before include "Restrict" , "Limit" , "distress" , "Train set (anag.)" .)