Bullock, for example, about to be held by a lock (7)

I believe the answer is:

actress

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'bullock for example' is the definition.
I can't tell whether this definition defines the answer.

'about to be held by a lock' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa'**).
'to be held by' indicates putting letters inside.
'lock' becomes 'tress' (I've seen this before).
'a'+'tress'='atress'
'c' put inside 'atress' is 'ACTRESS'.

Can you help me to learn more?

(Other definitions for actress that I've seen before include "Female player" , "Recasts (anag) - performer" , "Casters (anag.)" , "Garland perhaps" , "Female performer" .)

I've seen this clue in The Telegraph.

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