Wager fish will be gravely disloyal (6)

I believe the answer is:

betray

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'be gravely disloyal' is the definition.
I can't judge whether this defines the answer.

'wager fish' is the wordplay.
'wager' becomes 'bet' (synonyms).
'fish' becomes 'ray' (ray is a kind of fish).
'bet'+'ray'='BETRAY'

'will' is the link.

Can you help me to learn more?

(Other definitions for betray that I've seen before include "26 [SELL]" , "Act treacherously towards (someone)" , "Double-cross like Judas" , "grass" , "Be a traitor" .)

I've seen this clue in The Irish Times.

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