Fluid mechanics causing accident (9)
I believe the answer is:
mischance
'accident' is the definition.
(I've seen this before)
'fluid mechanics' is the wordplay.
'fluid' indicates an anagram.
'mechanics' with letters rearranged gives 'MISCHANCE'.
'causing' acts as a link.
(Other definitions for mischance that I've seen before include "Ill fortune" , "Misadventure, mishap" , "Unfortunate occurrence" , "Unfortunate business" , "Bad luck" .)