The pest that wrecked a ship (5)
I believe the answer is:
aphis
'the pest' is the definition.
(I've seen this before)
'wrecked a ship' is the wordplay.
'wrecked' is an anagram indicator.
'a'+'ship'='aship'
'aship' anagrammed gives 'APHIS'.
'that' acts as a link.
(Other definitions for aphis that I've seen before include "Small insect" , "sucker?" , "Plant louse" , "Sap-sucking garden pest" , "Greenfly" .)