Colour that is constant in salt water (5)
I believe the answer is:
sepia
'colour' is the definition.
(I know that sepia is a type of pigment)
'constant in salt water' is the wordplay.
'constant' becomes 'pi' (mathematics: 3.14).
'in' means one lot of letters goes inside another.
'salt water' becomes 'sea' (the sea is made up of salt water).
'pi' inserted inside 'sea' is 'SEPIA'.
'that is' acts as a link.
(Other definitions for sepia that I've seen before include "Denizen of deep" , "Reddish-brown colour as in old photos" , "Colourant" , "Brown of old photos" , "Dark reddish-brown pigment" .)