Passing rain's unfortunately penetrating temporary shelter (9)
I believe the answer is:
transient
'passing' is the definition.
('transient' can be a synonym of 'passing')
'rain's unfortunately penetrating temporary shelter' is the wordplay.
'unfortunately' is an anagram indicator.
'penetrating' means one lot of letters goes inside another.
'temporary shelter' becomes 'tent' (I've seen this before).
'rains' with letters rearranged gives 'ransi'.
'ransi' inserted into 'tent' is 'TRANSIENT'.
(Other definitions for transient that I've seen before include "Temporary worker" , "on the move" , "Here today, gone tomorrow" , "Lasting a short time" , "Fleeting" .)