Ride taken round Honshu, principally, by inscrutable person (6)
I believe the answer is:
sphinx
'inscrutable person' is the definition.
(I've seen this before)
'ride taken round honshu principally by' is the wordplay.
'ride' becomes 'spin'.
'taken round' indicates putting letters inside.
'principally' suggests taking the first letters.
'by' becomes 'x' (multiplication sign).
The first letter of 'honshu' is 'h'.
'spin' enclosing 'h' is 'sphin'.
'sphin'+'x'='SPHINX'
(Other definitions for sphinx that I've seen before include "Monster of Egyptian statuary" , "'Riddling monster, half-woman, half-lion (6)'" , "Egypt. monster statue" , "Monster - Egyptian statue" , "mythical monster" .)