Variable level holds key when reversing fundamental theory (10)
I believe the answer is:
relativity
'fundamental theory' is the definition.
(theory of relativity)
'variable level holds key when reversing' is the wordplay.
'variable' becomes 'y' (algebra).
'level' becomes 'tier' (synonyms).
'holds' indicates putting letters inside.
'key' becomes 'vital' (I've seen this before).
'when reversing' is a reversal indicator.
'y'+'tier'='ytier'
'ytier' going around 'vital' is 'ytivitaler'.
'ytivitaler' written backwards gives 'RELATIVITY'.
(Another definition for relativity that I've seen is " scientific principle".)