Part of the railroad I’d sing about (6)
I believe the answer is:
Here is my best explanation:
'part of the railroad' is the definition.
'siding' can be an answer for 'railroad' (thesaurus). I am not sure about the 'part of the' bit.
'i'd sing about' is the wordplay.
'about' indicates putting letters inside.
'id' put inside 'sing' is 'SIDING'.
(Other definitions for siding that I've seen before include "passing track", "Short railway branch", "Not the main line", "section", "Where coaches may be", "Short stretch of railroad track used for shunting", "short railway track", "Line", "location for shunter" and "Railway marshalling area".)