Photographed piece that can encourage reluctant union (7)

I believe the answer is:

shotgun

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'can encourage reluctant union' is the definition.
I can't tell whether this definition defines the answer.

'photographed piece' is the wordplay.
'photographed' becomes 'shot' ('shoot' can be a synonym of 'photograph').
'piece' becomes 'gun' (slang term for a gun).
'shot'+'gun'='SHOTGUN'

'that' acts as a link.

Can you help me to learn more?

(Other definitions for shotgun that I've seen before include "Firearm used at short range - kind of wedding" , "Hunting firearm" , "Noughts (anag) - type of wedding" , "No thugs (anag) - weapon" , "arm" .)

I've seen this clue in The Guardian.

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