Colour of credibility (3)
I believe the answer is:
red
'colour' is the definition.
(I know that red is a type of colour)
'of credibility' is the wordplay.
'of' says the answer is hidden in the clue.
'RED' can be found hidden inside 'credibility'.
(Other definitions for red that I've seen before include "Bloodshot" , "Scarlet or vermilion" , "- - poker: Kniphofia" , "Colour for danger" , "Colour of 1 across" .)