Seasick in Hertfordshire town? That's stunningly shocking! (8)
I believe the answer is:
tasering
'stunningly shocking' is the definition.
(taser is an electric shock weapon)
'seasick in hertfordshire town?' is the wordplay.
'seasick' becomes 'ase' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'hertfordshire town?' becomes 'tring' (I've seen this in other clues).
'ase' inserted inside 'tring' is 'TASERING'.
'that's' acts as a link.
(Other definitions for tasering that I've seen before include "Shocking criminal" , "Using a shocking weapon" .)