A bit of indigo resembles blood (4)
I believe the answer is:
gore
'blood' is the definition.
(gore means blood or violence)
'a bit of indigo resembles' is the wordplay.
'a bit of' indicates a hidden word.
'GORE' can be found hidden inside 'indigo resembles'.
(Other definitions for gore that I've seen before include "Result of bleeding" , "Blood that's been shed" , "piece of material" , "Impale" , "stuff from part of Kensington" .)