Sharp American hero (4)
I believe the answer is:
acid
'sharp' is the definition.
(as in sharp-tasting)
'american hero' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'hero' becomes 'cid' (El Cid, Spanish military hero).
'a'+'cid'='ACID'
(Other definitions for acid that I've seen before include "Battery fluid" , "something sharp" , "Sour, sharp-tasting" , "Sour - type of rain or drop" , "Morose" .)