Mass and energy extracted from a number having strength (6)
I believe the answer is:
mighty
'having strength' is the definition.
'mass and energy extracted from a number' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'and' says to put letters next to each other.
'energy' becomes 'e' (physics symbol as in E=mc2).
'extracted from' is a deletion indicator.
'a number' becomes 'eighty' (eighty is a kind of number).
'eighty' with 'e' removed is 'ighty'.
'm'+'ighty'='MIGHTY'
(Other definitions for mighty that I've seen before include "Powerful - exalted" , "Powerful and great" , "Very powerful/strong" , "Powerful - huge" , "Arrogant people are often described as high and . . . . . ." .)