Fluid line, sadly invalidated (9)
I believe the answer is:
nullified
'invalidated' is the definition.
(synonyms)
'fluid line sadly' is the wordplay.
'sadly' indicates an anagram.
'fluid'+'line'='fluidline'
'fluidline' with letters rearranged gives 'NULLIFIED'.
(Other definitions for nullified that I've seen before include "Cancelled out" , "Made invalid" , "Abrogated" , "Scrubbed" , "revoked" .)