Of purely theoretical interest (8)
I believe the answer is:
academic
'of purely theoretical interest' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for academic that I've seen before include "Theoretical or scholarly - often not practical" , "Of theoretical interest, no practical application" , "Not expected to produce any practical result" , "Irrelevant; teacher" , "Well-read" .)