In plain English
FAR 52.247-12, Supervision, Labor, or Materials, is a contract clause prescribed at FAR 47.207-5(b), most recently dated Apr 1984. The complete official text is reproduced below, verbatim, from GSA's published FAR source files.
Its text contains no sentence requiring insertion into subcontracts; see the flowdown section below for what that does and does not mean.
Does it flow down to subcontracts?
No flowdown mandate found in the clause text
We scanned the full clause text and found no sentence directing the contractor to insert this clause into subcontracts. That is a statement about the text, not legal advice: a prime contractor may still flow terms down contractually, an agency supplement (DFARS, VAAR, …) may add requirements, and clauses listed below (if any) may order this clause into subcontracts from the outside.
Where it's prescribed
As prescribed in 47.207-5(b), insert a clause substantially as follows in solicitations and contracts for transportation or for transportation-related services when the contractor is required to furnish supervision, labor, or materials:
Prescribing reference: FAR 47.207-5(b).
The official text, verbatim
As prescribed in 47.207-5(b), insert a clause substantially as follows in solicitations and contracts for transportation or for transportation-related services when the contractor is required to furnish supervision, labor, or materials:
Supervision, Labor, or Materials (Apr 1984)
The Contractor shall furnish adequate supervision, labor, materials, supplies, and equipment necessary to perform all the services contemplated under this contract in an orderly, timely, and efficient manner.
(End of clause)