Unfortunate occurrence in fluid mechanics (9)
I believe the answer is:
'unfortunate occurrence' is the definition.
(I've seen this before)
'fluid mechanics' is the wordplay.
'fluid' is an anagram indicator.
'mechanics' anagrammed gives 'MISCHANCE'.
'in' is the link.
(Other definitions for mischance that I've seen before include "Ill fortune" , "Bad luck" , "Misadventure, mishap" , "An accident of mechanics" , "Unfortunate occurrence" .)