Pretended enthusiasm about karate, losing heart (5)
I believe the answer is:
faked
'pretended' is the definition.
(faking is a kind of pretending)
'enthusiasm about karate losing heart' is the wordplay.
'enthusiasm' becomes 'fad' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' is an insertion indicator.
'losing heart' means to remove the middle letters.
'karate' with its centre taken out is 'ke'.
'fad' placed around 'ke' is 'FAKED'.
Can you help me to learn more?
(Another definition for faked that I've seen is " Counterfeit".)