Shade of credibility (3)
I believe the answer is:
red
'shade' is the definition.
(red is a colour or shade)
'of credibility' is the wordplay.
'of' indicates the answer is hidden within the clue.
'RED' can be found hidden inside 'credibility'.
(Other definitions for red that I've seen before include "Colour of Merlot wine" , "- Cross; - admiral" , "Type of wine" , "Bordeaux claret, for instance" , "Flushed" .)