Line number up on step briefly in city once (9)
I believe the answer is:
leningrad
'city once' is the definition.
(former name of St Petersburg)
'line number up on step briefly' is the wordplay.
'line number up' becomes 'lenin' (I can't explain this - if you can you should believe this answer much more).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'step' becomes 'grade'.
'briefly' means to remove the last letter.
'grade' with its last letter taken off is 'grad'.
'lenin'+'grad'='LENINGRAD'
'in' is the link.
Can you help me to learn more?
(Other definitions for leningrad that I've seen before include "Name of St Petersburg from 1924 to 1991" , "Former name of city" , "St Petersburg, 1924-91" , "revolutionary city" , "Soviet period name for St Petersburg" .)