Pier collapsing - 50 in danger (5)
I believe the answer is:
peril
'in danger' is the definition.
(I've seen this before)
'pier collapsing 50' is the wordplay.
'collapsing' indicates an anagram.
'50' becomes 'l' (roman numeral).
'pier' anagrammed gives 'peri'.
'peri'+'l'='PERIL'
(Other definitions for peril that I've seen before include "Immediate danger" , "Quest" , "risk" , "Danger, hazard" , "Danger of serious kind" .)