Having a sound relationship with a boy, I see (5)

I believe the answer is:

sonic

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'having a sound relationship' is the definition.
I can't judge whether this defines the answer.

'with a boy i see' is the wordplay.
'with' says to put letters next to each other.
'a boy' becomes 'son' (synonyms).
'see' becomes 'C' (the word for the letter, according to Chambers).
'son'+'i' is 'soni'.
'soni'+'c'='SONIC'

Can you help me to learn more?

(Other definitions for sonic that I've seen before include "Relating to sound waves" , "seven hundred and forty three mph!" , "Boom of jet aircraft" , "of noise" , "Travelling at Mach One" .)

I've seen this clue in the Evening Standard.

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