Union representative's accommodating on the subject of space (9)
I believe the answer is:
agreement
'union' is the definition.
(I've seen this before)
'representative's accommodating on the subject of space' is the wordplay.
'representative' becomes 'agent' (agent is a kind of representative).
'accommodating' means one lot of letters goes inside another.
'on the subject of' becomes 're' (re means regarding a certain topic).
'space' becomes 'em' (in typography, a space as wide as a letter 'm').
're'+'em'='reem'
'agent' placed around 'reem' is 'AGREEMENT'.
(Other definitions for agreement that I've seen before include "Concord, harmony" , "understanding" , "Conformity" , "Contract or promise" , "consensus" .)