Short-lived rain's poured in many a time (9)
I believe the answer is:
transient
'short-lived' is the definition.
(I know that short-lived can be written as transient)
'rain's poured in many a time' is the wordplay.
'poured' indicates anagramming the letters (letters get poured or mixed around).
'in' is an insertion indicator.
'many' becomes 'ten' (example).
'a time' becomes 't' (abbreviation).
'rains' is an anagram of 'ransi'.
'ransi' put into 'ten' is 'transien'.
'transien'+'t'='TRANSIENT'
(Other definitions for transient that I've seen before include "on the move" , "Passing, short-lived" , "fugitive" , "gone in a trice!" , "Here today, gone tomorrow" .)