Endless nubility vigorously essential (7)
I believe the answer is:
inbuilt
'essential' is the definition.
(I've seen this before)
'endless nubility vigorously' is the wordplay.
'endless' means to remove the last letter.
'vigorously' indicates an anagram (the letters move vigorously around).
'nubility' with its final letter taken away is 'nubilit'.
'nubilit' with letters rearranged gives 'INBUILT'.
(Other definitions for inbuilt that I've seen before include "essential" , "Integral" , "Being an original feature" , "Native" , "Fitted" .)