Single flat by railway (7)

I believe the answer is:

unitary

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'single' is the definition.
(I've seen this before)

'flat by railway' is the wordplay.
'flat' becomes 'unita' (I am not sure about this - if you are sure you should believe this answer much more).
'by' says to put letters next to each other.
'railway' becomes 'ry' (abbreviation).
'unita'+'ry'='UNITARY'

Can you help me to learn more?

(Other definitions for unitary that I've seen before include "Single; uniform" , "as a whole" , "Marked by oneness" , "showing one nature" .)

I've seen this clue in the Sydney Morning Herald.

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