Swaying pier left causes danger (5)
I believe the answer is:
peril
'danger' is the definition.
(synonyms)
'swaying pier left' is the wordplay.
'swaying' indicates an anagram (letters swaying or moving around).
'left' becomes 'L' (common abbreviation).
'pier' with letters rearranged gives 'peri'.
'peri'+'l'='PERIL'
'causes' is the link.
(Other definitions for peril that I've seen before include "risk" , "Source of danger" , "Immediate danger" , "Exposure to harm" , "Quest" .)