Accident causing mass destruction at Holyhead (5)
I believe the answer is:
smash
'accident' is the definition.
(smash is a kind of accident)
'mass destruction at holyhead' is the wordplay.
'destruction' indicates anagramming the letters.
'at' says to put letters next to each other.
'holyhead' becomes 'h' (head letter of 'holy').
'mass' is an anagram of 'smas'.
'smas'+'h'='SMASH'
'causing' acts as a link.
(Other definitions for smash that I've seen before include "defeat" , "Overhead tennis shot" , "Break noisely into pieces" , "Hard tennis shot" , "Shatter, break up" .)