Risk adopting American size of paper (8)
I believe the answer is:
imperial
'size of paper' is the definition.
(size of paper which is roughly 22 by 30 inches)
'risk adopting american' is the wordplay.
'risk' becomes 'imperil' (I've seen this before).
'adopting' is an insertion indicator.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'imperil' placed around 'a' is 'IMPERIAL'.
(Other definitions for imperial that I've seen before include "Relating to empire" , "Sovereign - old Russian gold coin - type of beard" , "I prime Al to describe the rule of great territories" , "To do with empire or emperor" , "Of UK weights and measures" .)