Green bananas the beginning of your vitality (6)
I believe the answer is:
energy
'vitality' is the definition.
(I know that vitality can be written as energy)
'green bananas the beginning of your' is the wordplay.
'bananas' indicates anagramming the letters (informal term for mad).
'the beginning of' suggests taking the first letters.
The initial letter of 'your' is 'y'.
'green' with letters rearranged gives 'energ'.
'energ'+'y'='ENERGY'
(Other definitions for energy that I've seen before include "Exertion of force" , "Healthy capacity for vigorous activity" , "Vigour, vim, vitality" , "It's measured in joules or ergs" , "Vigour, spirit" .)