Colour that is constant in salt water (5)
I believe the answer is:
sepia
'colour' is the definition.
(sepia is a kind of colour)
'constant in salt water' is the wordplay.
'constant' becomes 'pi' (mathematics: 3.14).
'in' indicates putting letters inside.
'salt water' becomes 'sea' (the sea is made up of salt water).
'pi' placed inside 'sea' is 'SEPIA'.
'that is' is the 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" .)