Crossword Genius

Son's promise in front of mainly suspicious swimmer (9)

Ross

I believe the answer is:

swordfish

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

'son's promise in front of mainly suspicious' is the wordplay.
'son's promise' becomes 'sword' (I can't justify this - if you can you should believe this answer much more).
'in front of' says to put letters next to each other.
'mainly' means to remove the last letter.
'suspicious' becomes 'fishy' (synonyms).
'fishy' with its last letter taken away is 'fish'.
'sword'+'fish'='SWORDFISH'

Can you help me to learn more?

(Other definitions for swordfish that I've seen before include "Xiphius gladius" , "Creature using its long jaw to stab" , "sea food" , "Large marine creature" , "(Edible) long-snouted swimmer" .)

I've seen this clue in The Telegraph.
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