Decrepit canon takes a tumble, being this? (8-5)
I believe the answer is:
accident-prone
'this?' is the definition.
The definition suggests a singular noun which matches the answer.
'decrepit canon takes a tumble' is the wordplay.
'takes a tumble' indicates anagramming the letters.
'decrepit'+'canon'='decrepitcanon'
'decrepitcanon' is an anagram of 'ACCIDENT-PRONE'.
'being' acts as a link.
(Other definitions for accident-prone that I've seen before include "Involved in unusually frequent mishaps" , "Clumsy" , "Susceptible to mishap" , "Unusually susceptible to mishaps" , "Liable to have mishaps" .)