Respond about facts lacking in case (5)
I believe the answer is:
react
'respond' is the definition.
(to react to something is to respond to it)
'about facts lacking in case' is the wordplay.
I cannot quite understand how this works, but
'about' could be 'rea' (I've seen this in other clues) and 'rea' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for react that I've seen before include "Trace (anag)" , "Change chemically" , "Respond to something" , "Retaliate" , "Take" .)