Back one aiming to retain his title (8)

I believe the answer is:

defender

I believe this clue is a double definition.

'back' is the first definition.
(I've seen this before)

'one aiming to retain his title' is the second definition.
(I've seen this before)

(Other definitions for defender that I've seen before include "supporter" , "Advocate" , "One who protects others" , "Protector" , "Centre half, perhaps" .)

I've seen this clue in The Globe and Mail, The Guardian and The Telegraph.

Want a hint initially instead of a full solution? Install my app