This is an IFF antenna that replaced steel wires going from tips of the horizontal stabilizers to sides of the fuselage near the radio compartment used with earlier marks of the Spitfire.
"In early 1943 a new IFF, the Mk III started being fitted to all front-line fighters. On the Spitfire the fuselage to tailplane wires were replaced by a single rod aerial mounted under the starboard wing. At about the same time the updated TR 1143 radio sets started replacing the TR 1133s, although no external changes were visible."