Free entertainment is essentially cheap work (8)
I believe the answer is:
treatise
'work' is the definition.
(I have seen 'formal work' mean 'treatise' so perhaps 'work' could also mean 'treatise')
'free entertainment is essentially cheap' is the wordplay.
'free entertainment' becomes 'treat' (I've seen this before).
'essentially' means to look at the middle letters.
The central letter of 'cheap' is 'e'.
'treat'+'is'+'e'='TREATISE'
(Other definitions for treatise that I've seen before include "Learned written work" , "Iterates in essay" , "Learned writing" , "Systematically-written formal piece" , "A written work about treaties" .)