Theoretically, often heartless treatise (2,5)
I believe the answer is:
on paper
'theoretically' is the definition.
('on paper' can be a synonym of 'theoretically')
'often heartless treatise' is the wordplay.
'heartless' says to hollow out the word (remove centre letters).
'treatise' becomes 'paper' (I've seen this before).
'often' with its middle removed is 'on'.
'on'+'paper'='ON PAPER'
(Other definitions for on paper that I've seen before include "Hypothetically" , "Planned (in theory)" , "As written - in theory" , "Propane (anag.)" , "Theoretically" .)