Allocating roles to defend line protecting the king (8)
I believe the answer is:
castling
'protecting the king' is the definition.
The answer and definition can be both related to competing as well as being present participle verbs.
Perhaps there's an association between them I don't understand?
'allocating roles to defend line' is the wordplay.
'allocating roles' becomes 'casting'.
'to defend' means one lot of letters goes inside another.
'line' becomes 'l' (used when specifying particular lines from a poem).
'casting' placed around 'l' is 'CASTLING'.
Can you help me to learn more?
(Other definitions for castling that I've seen before include "defence of the board" , "Moving two men on board" , "chess move" .)