Horrified - has the wrong tag showing outside (6)
I believe the answer is:
aghast
'horrified' is the definition.
(I know that horrified can be written as aghast)
'has the wrong tag showing outside' is the wordplay.
'the wrong tag showing' becomes 'agt' (I can't explain this - if you can you should believe this answer much more).
'outside' is an insertion indicator.
'has' inserted within 'agt' is 'AGHAST'.
Can you help me to learn more?
(Other definitions for aghast that I've seen before include "Taken aback with horror" , "'Appalled, dismayed (6)'" , "Amazed" , "Stupefied with horrer" , "Deeply dismayed" .)