If you want to take something up, now's the time to choose (5)
I believe the answer is:
adopt
'if you want to take something up now's' is the definition.
I can't tell whether this definition defines the answer.
'time to choose' is the wordplay.
'time' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'to choose' becomes 'opt' (both can mean to pick something).
'ad'+'opt'='ADOPT'
'the' is the link.
Can you help me to learn more?
(Other definitions for adopt that I've seen before include "Espouse" , "Choose as one's own" , "Start to wear" , "choose as candidate" , "Take up, follow" .)