Reason to go round the corner in a passage (6)
I believe the answer is:
clause
'passage' is the definition.
(thesaurus)
'reason to go round the corner' is the wordplay.
'reason' becomes 'cause' (I've seen this before).
'to go round' indicates putting letters inside.
'the corner' becomes 'l' (I am not sure about this - if you are sure you should believe this answer much more).
'cause' going around 'l' is 'CLAUSE'.
'in a' acts as a link.
Can you help me to learn more?
(Other definitions for clause that I've seen before include "Not quite a full sentence" , "Subdivision of a sentence" , "Bill's article" , "Separate item in contract" , "Part of sentence or document" .)