In the middle of telecast, callipers used to block out light (7)

I believe the answer is:

eclipse

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'block out light' is the definition.
'eclipse' can be an answer for 'block' (I've seen this before). I'm unsure of the 'out light' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for eclipse that I've seen before include "Solar/lunar phenomenon" , "Blot out" , "Dull time for male duck" , "blackout" , "Outdo" .)

I've seen this clue in The Irish Times.

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