It's forbidden to wreck a boat on the 1st of October (5)
I believe the answer is:
taboo
'it's forbidden' is the definition.
(synonyms)
'wreck a boat on the 1st of october' is the wordplay.
'wreck' is an anagram indicator.
'on' indicates putting letters inside (as in clothing 'on' a person).
'the 1st of october' becomes 'o' (1st letter of 'october').
'boat' with letters rearranged gives 'tabo'.
'tabo' enclosing 'o' is 'TABOO'.
'to' acts as a link.
(Other definitions for taboo that I've seen before include "Socially forbidden" , "Not permitted" , "Prohibited as unclean or holy" , "No-no" , "Beyond the pale" .)