Writers' second copies' entertaining energy (7)
I believe the answer is:
scribes
'writers'' is the definition.
(scribe is a kind of writer)
'second copies' entertaining energy' is the wordplay.
'second' becomes 's' (common abbreviation).
'copies'' becomes 'cribs'.
'entertaining' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'cribs' enclosing 'e' is 'cribes'.
's'+'cribes'='SCRIBES'
(Other definitions for scribes that I've seen before include "Copyists" , "Ancient writers (7)" , "People who once made hand-written copies" , "journos" .)