You need to be quick to take one (6)
I believe the answer is:
breath
'one' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'you need to be quick to take' is the wordplay.
'you need' becomes 'brea' (I am not sure about this - if you are sure you should believe this answer much more).
'to be' becomes 'are' (I've seen this in another clue).
'quick' becomes 'rathe'.
'to take' indicates named letters should be taken away.
'rathe' with 'are' removed is 'th'.
'brea'+'th'='BREATH'
Can you help me to learn more?
(Other definitions for breath that I've seen before include "Air that's inhaled and exhaled" , "Respiration" , "Air in an out of the lungs" , "Bather (anag.)" , "from which inspiration may be drawn" .)