Classified source of energy found in new crop test (3,6)
I believe the answer is:
top secret
'classified' is the definition.
('top-secret' can be similar in meaning to 'classified')
'source of energy found in new crop test' is the wordplay.
'source of' indicates taking the first letters.
'found in' means one lot of letters goes inside another.
'new' indicates an anagram.
The initial letter of 'energy' is 'e'.
'crop'+'test'='croptest'
'croptest' is an anagram of 'topsecrt'.
'e' put within 'topsecrt' is 'TOP-SECRET'.
(Other definitions for top secret that I've seen before include "label on some classified documents" , "Highly confidential" , "restricted" , "As 007 mission" , "(Of information) strictly not for public consumption" .)