Useless revolutionary academic full of sentimentality (2,4)
I believe the answer is:
no good
'useless' is the definition.
(I've seen this before)
'revolutionary academic full of sentimentality' is the wordplay.
'revolutionary' says the letters should be written backwards.
'academic' becomes 'don' (I've seen this before).
'full of' indicates putting letters inside.
'sentimentality' becomes 'goo' (informal term for sentimentality).
'don' backwards is 'nod'.
'nod' going around 'goo' is 'NO-GOOD'.
(Other definitions for no good that I've seen before include "Useless, not worth bothering about" , "Hopeless" , "(Of a person) beneath contempt" , "worthless sort" , "Futile; worthless" .)