Change poles for stopping streetcar (9)
I believe the answer is:
transform
'change' is the definition.
(transforming is a kind of changing)
'poles for stopping streetcar' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'streetcar' becomes 'tram' ('tram' can be a synonym of 'streetcar').
'ns'+'for'='nsfor'
'nsfor' going into 'tram' is 'TRANSFORM'.
(Other definitions for transform that I've seen before include "Remodel" , "Subject to change" , "Change completely" , "Make great change in appearance of" , "Change the voltage I" .)