Old adult in vain working without energy (12)
I believe the answer is:
antediluvian
'old' is the definition.
('antediluvian' can be similar in meaning to 'old')
'adult in vain working without energy' is the wordplay.
'working' is an anagram indicator.
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'energy' becomes 'e' (physics symbol as in E=mc2).
'adult'+'in'+'vain'='adultinvain'
'adultinvain' anagrammed gives 'antdiluvian'.
'antdiluvian' placed around 'e' is 'ANTEDILUVIAN'.
(Other definitions for antediluvian that I've seen before include "Before the flood; ancient" , "Very old (before the Flood)" , "Very old-fashioned (from before the Flood?)" , "before the water's bubbled up" .)