Passing rain's oddly penetrating shelter (9)
I believe the answer is:
transient
'passing' is the definition.
(synonyms)
'rain's oddly penetrating shelter' is the wordplay.
'oddly' indicates anagramming the letters (letters in an odd order).
'penetrating' means one lot of letters goes inside another.
'shelter' becomes 'tent' (a tent is a kind of shelter).
'rains' is an anagram of 'ransi'.
'ransi' inserted into 'tent' is 'TRANSIENT'.
(Other definitions for transient that I've seen before include "fugitive" , "Temporary worker" , "on the move" , "Here today, gone tomorrow" , "Ephemeral" .)