Length — short one extending end of railway (3)
I believe the answer is:
ell
'length' is the definition.
(I've seen this in other clues)
'short one extending end of railway' is the wordplay.
'short' says to take the initial letters.
'end of railway' becomes 'll' (I am not sure about this - if you are sure you should believe this answer much more).
The initial letter of 'extending' is 'e'.
'e'+'ll'='ELL'
(Other definitions for ell that I've seen before include "Extension" , "Former textile measure" , "Old measure of length about 45 inches" , "Old measure equivalent to about 45 inches in England" , "Old 45-inch measure" .)