On a rickety pier, a number are in danger (5)
I believe the answer is:
peril
'in danger' is the definition.
(I've seen this before)
'on a rickety pier a number' is the wordplay.
'on' means one lot of letters go next to another.
'a rickety' indicates anagramming the letters.
'a number' becomes 'l' (fifty in Roman numerals).
'pier' is an anagram of 'peri'.
'peri' put next to 'l' is 'PERIL'.
'are' acts as a link.
(Other definitions for peril that I've seen before include "Source of danger" , "Exposure to harm" , "Danger, hazard" , "Danger of serious kind" , "risk" .)