She expresses surprise about American gun (6)
I believe the answer is:
agatha
'she' is the definition.
'surprise about american gun' is the wordplay.
'surprise' becomes 'ha' (I've seen this in other clues).
'about' is a charade indicator (letters next to each other).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'gun' becomes 'gat' (US informal term for a gun).
'a'+'gat'='agat'
'ha' after 'agat' is 'AGATHA'.
'expresses' acts as a link.
(Other definitions for agatha that I've seen before include "Dame - Christie, English crime writer" , "Detective story writer" , "...... Christie wrote great detective stories" , "-- Christie, English novelist" , "Aunt of Wooster" .)