He's round the bend, just a shade! (3)
I believe the answer is:
hue
'a shade' is the definition.
(both can mean a colour)
'he's round the bend' is the wordplay.
'round' means one lot of letters goes inside another.
'the bend' becomes 'U' (a U-bend is a kind of bend - pipe under sink).
'he' placed around 'u' is 'HUE'.
'just' is the link.
(Other definitions for hue that I've seen before include "Tinge" , "Tint, shade" , "Cast" , "Shade, aspect; Vietnam port" , "General outburst of alarm or protest - . . . and cry" .)