Speak falsely about stamina (5)
I believe the answer is:
fibre
'speak falsely about stamina' is the definition.
I can't tell whether this definition defines the answer.
'speak falsely about' is the wordplay.
'speak falsely' becomes 'fib' (I can't explain this - if you can you should give a lot more credence to this answer).
'about' becomes 're' (regarding).
'fib'+'re'='FIBRE'
Can you help me to learn more?
(Other definitions for fibre that I've seen before include "(Material composed of) thread(s)" , "Thread - strength of character" , "Coarse plant food, low in nutrients" , "Textile thread; roughage" , "Dietary material; thread" .)