In prison cubicle at Seoul, lost some rayon base (9)
I believe the answer is:
But I can't explain it at all! Could you help me understand?
(Other definitions for cellulose that I've seen before include "O cull eels for what causes the pitted look", "Polysaccharide", "dietary fibre", "film-makers use it", "Substance in plants -- so cull eel (anag)".)
I've seen this clue in Sydney Morning Herald.