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' says the answer is hidden in the clue.
'RED' is hidden within '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" .)