Channel Seven's principal characteristic (6)

I believe the answer is:

strait

'channel' is the definition.
(strait is a kind of channel)

'seven's principal characteristic' is the wordplay.
'principal' suggests taking the first letters.
'characteristic' becomes 'trait' (I've seen this before).
The initial letter of 'seven' is 's'.
's'+'trait'='STRAIT'

(Other definitions for strait that I've seen before include "Narrow sea-channel" , "Narrow sea passage" , "Difficulty - strict" , "watery scene?" , "of Gibraltar, for example" .)

I've seen this clue in The Times.

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