Stunned seeing a grand head of state in stetson? (6)
I believe the answer is:
aghast
'stunned' is the definition.
(thesaurus)
'a grand head of state in stetson?' is the wordplay.
'a grand' becomes 'ag' (I've seen this in other clues).
'head of' indicates taking the first letters.
'in' indicates putting letters inside.
'stetson?' becomes 'hat' (stetson is a kind of hat).
The first letter of 'state' is 's'.
's' put inside 'hat' is 'hast'.
'ag'+'hast'='AGHAST'
'seeing' is the link.
(Other definitions for aghast that I've seen before include "Horrified, appalled" , "Struck with horror" , "Filled with fear and consternation" , "'Horrified, shocked (6)'" , "Horror-struck" .)