Turns west first, into a back street (6)
I believe the answer is:
twists
'turns' is the definition.
(I know this)
'west first into a back street' is the wordplay.
'west' becomes 'W' (abbreviation).
'first' becomes 'ist' (ist resembles 1st).
'into' indicates putting letters inside.
'a back' says the letters should be written in reverse.
'street' becomes 'st' (abbreviation).
'st' in reverse letter order is 'ts'.
'w'+'ist'='wist'
'wist' put within 'ts' is 'TWISTS'.
(Other definitions for twists that I've seen before include "Wrenches round" , "gets another card turned over" , "Winds, turns around" , "Rotates; distorts" , "Interweaves or gives spiral form to" .)