It made old people look blue (4)
I believe the answer is:
woad
'blue' is the definition.
(thesaurus)
'it made old people look' is the wordplay.
I cannot really understand how this works, but
'old' could be 'o' (abbreviation as in OT - Old Testament) and 'o' is present in the answer.
The remaining letters 'wad' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for woad that I've seen before include "Blue dyestuff" , "Plant yielding blue dye" , "Old British blue dye" , "Colouring" , "Ancient blue dye" .)