Promise broken by Labour — “We’re all in it together” (5)
I believe the answer is:
world
'we're all in it together' is the definition.
(I've seen this in another clue)
'promise broken by labour' is the wordplay.
'promise' becomes 'word' (as in 'I give you my word').
'broken by' is an insertion indicator (inserted letters break into the word).
'labour' becomes 'L' (political abbreviation).
'word' enclosing 'l' is 'WORLD'.
(Other definitions for world that I've seen before include "Everyone" , "Field" , "Cosmos" , "Everything that exists anywhere" , "The globe" .)