American guy, lacking energy, is a failure (3)
I believe the answer is:
dud
'is a failure' is the definition.
(I've seen this before)
'american guy lacking' is the wordplay.
'american guy' becomes 'dude'.
'lacking' means to remove the last letter (the word is incomplete or lacking).
'dude' with its final letter removed is 'DUD'.
'energy' is the link.
This may not be correct. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dud that I've seen before include "Something useless - a palindrome" , "A thing failing to work" , "Useless, ineffective" , "Broken" , "Defective" .)