Supported by leaders of union, picket office next (4)

I believe the answer is:

upon

'supported by' is the definition.
(I've seen this before)

'leaders of union picket office next' is the wordplay.
'leaders of' indicates taking the first letters.
The initial letters of 'union picket office next' is 'UPON'.

(Other definitions for upon that I've seen before include "Atop" , ".... my word! I'm shocked" , "Onto" , "In contact with" , "(Placed) on top of" .)

I've seen this clue in The Telegraph.

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