They can see half a dozen vessels (7)

I believe the answer is:

viewers

'they can see' is the definition.

'half a dozen vessels' is the wordplay.
'half a dozen' becomes 'vi' ('vi' can be a synonym of 'half a dozen').
'vessels' becomes 'ewers' (ewer is a kind of vessel).
'vi'+'ewers'='VIEWERS'

(Other definitions for viewers that I've seen before include "TV addicts" , "spectators" .)

I've seen this clue in The Times.

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