Brown, private investigator, seen in water (5)
I believe the answer is:
sepia
'brown' is the definition.
(I know that sepia is a type of brown)
'private investigator seen in water' is the wordplay.
'private investigator' becomes 'PI' (abbreviation).
'seen in' means one lot of letters goes inside another.
'water' becomes 'sea' (the sea is water).
'pi' going inside 'sea' is 'SEPIA'.
(Other definitions for sepia that I've seen before include "Denizen of deep" , "brownish colour" , "Reddish-brown; cuttlefish ink" , "Reddish-brown colour - easy as pie" , "Colourant" .)