Stupefied American enjoys tucking into GT (6)
I believe the answer is:
aghast
'stupefied' is the definition.
(I've seen this before)
'american enjoys tucking into gt' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'enjoys' becomes 'has' (I've seen this in another clue).
'tucking into' indicates putting letters inside.
'has' put into 'gt' is 'ghast'.
'a'+'ghast'='AGHAST'
(Other definitions for aghast that I've seen before include "Stupefied with horrer" , "Filled with fear and consternation" , "What's carried in bag has teacher" , "Horror-struck" , "Appalled, filled with consternation" .)