Thrive as union enters into a contract (6)
I believe the answer is:
abound
'thrive' is the definition.
(thesaurus)
'union enters into a contract' is the wordplay.
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'enters into' indicates putting letters inside.
'contract' becomes 'bond' (I've seen this before).
'a'+'bond'='abond'
'u' going inside 'abond' is 'ABOUND'.
'as' is the link.
(Other definitions for abound that I've seen before include "Be plentiful, swarm" , "Exist in large numbers" , "rich" , "Exist in large quantities" , "grow on trees" .)