The man drowning in strange fluid (5)
I believe the answer is:
'fluid' is the definition.
(I've seen this before)
'the man drowning in strange' is the wordplay.
'the man' becomes 'he' (he refers to a man).
'drowning in' is an insertion indicator.
'strange' becomes 'rum' (synonyms).
'he' going within 'rum' is 'RHEUM'.
(Other definitions for rheum that I've seen before include "Watery discharge from eyes or nose" , "Rhubarb genus; a head cold" , "Catarrh" , "Watery mucous discharge" , "drips from the nose" .)