Shower was lined beneath exterior gaslight (9)
I believe the answer is:
brainwash
'shower was lined beneath exterior gaslight' is the definition.
I can't tell whether this definition defines the answer.
'shower was lined beneath exterior' is the wordplay.
'shower' becomes 'rain' (synonyms).
'lined beneath' becomes 'bh' (I can't explain this - if you can you should believe this answer much more).
'exterior' means one lot of letters goes inside another (some letters go on the outside of others).
'rain'+'was'='rainwas'
'rainwas' going into 'bh' is 'BRAINWASH'.
Can you help me to learn more?
(Other definitions for brainwash that I've seen before include "Condition" , "clean mind?" , "Coerce into belief" , "Completely indoctrinate" , "Subject someone to stressful indoctrination" .)