Sprightly but vacuous (4)
I believe the answer is:
spry
This might be a double definition.
'sprightly' is the first definition.
(I've seen this in another clue)
'vacuous' is the second definition.
Both the answer and definition are adjectives. Perhaps you can see a link between them that I can't see?
'but' is the link.
Can you help me to learn more?
(Other definitions for spry that I've seen before include "Fleet" , "Light on one's feet" , "Nimble, brisk" , "Far from doddering" , "Lively (especially of an older person)" .)