Legal agreement, one dealing with labour movement? (11)
I believe the answer is:
contraction
'labour movement?' is the definition.
'legal agreement one dealing with' is the wordplay.
'legal agreement' becomes 'contract' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'dealing with' becomes 'on'.
'contract'+'i'+'on'='CONTRACTION'
(Other definitions for contraction that I've seen before include "Process of size reduction" , "Isn't, for instance" .)