Flag measure that will determine the type of rolling stock they can use (8,5)
I believe the answer is:
standard gauge
'flag measure that will determine the type of rolling stock they can use' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?
'flag measure' is the wordplay.
I cannot really see how this works, but
'flag' could be 'standard' (standard is a kind of flag) and 'standard' is found within the answer.
'measure' could be 'gauge' (I've seen this before) and 'gauge' is found in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for standard gauge that I've seen before include "UK railway track width" , "Kind of railway" , "Normal railway track width" , "rail measure" , "Width between rails in UK" .)