Part of veteran thematic song? (6)
I believe the answer is:
Here is my best explanation:
'song?' is the definition.
(anthem is a kind of song)
'part of veteran thematic' is the subsidiary indication.
'part of' says the answer is hidden in the clue.
'anthem' is hidden within 'veteran thematic'.
(Other definitions for anthem that I've seen before include "Rousing song", "Religious chant", "National melody", "air of patriotism?", "National tune", "Patriotic song", "The man for a rousing song", "Church song", "Uplifting music" and "The man for a patriotic song".)