Flat lacks energy or vitality (8)
I believe the answer is:
lifeless
'flat' is the definition.
(thesaurus)
'lacks energy or vitality' is the wordplay.
I cannot quite see how this works, but
'energy' could be 'life' (I've seen this in another clue) and 'life' is found within the answer.
The remaining letters 'less' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for lifeless that I've seen before include "Passive" , "Dull" , "lacking any vitality" , "Inanimate or dead" , "Showing no signs of vitality" .)