Pigment good in water (5)
I believe the answer is:
sepia
'pigment' is the definition.
(I know that sepia is a type of pigment)
'good in water' is the wordplay.
'good' becomes 'pi' (old-fashioned term for pious).
'in' indicates putting letters inside.
'water' becomes 'sea' (the sea is water).
'pi' inserted into 'sea' is 'SEPIA'.
(Other definitions for sepia that I've seen before include "Brown pigment produced from cuttlefish ink" , "Brown colour of old photo" , "Denizen of deep" , "Brown tone associated with old photographs" , "Colourant" .)