Relationship left fluid (4)
I believe the answer is:
link
'relationship' is the definition.
(thesaurus)
'left fluid' is the wordplay.
'left' becomes 'l' (common abbreviation).
'fluid' becomes 'ink' (ink is a kind of fluid).
'l'+'ink'='LINK'
(Other definitions for link that I've seen before include "website feature" , "Make a connection" , "Tie-up" , "section of chain" , "Bond" .)