Employ union, spreading its lie (7)
I believe the answer is:
utilise
'employ' is the definition.
(synonyms)
'union spreading its lie' is the wordplay.
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'spreading' indicates anagramming the letters.
'its'+'lie'='itslie'
'itslie' is an anagram of 'tilise'.
'u'+'tilise'='UTILISE'
(Other definitions for utilise that I've seen before include "Put to use" , "Put into service, find application for" , "Employ, make use of" , "Employ, do something with" , "Find application or task for" .)