Rider has reason to drink litres (6)
I believe the answer is:
clause
'rider' is the definition.
(I've seen this in another clue)
'reason to drink litres' is the wordplay.
'reason' becomes 'cause' (I've seen this in another clue).
'to drink' is an insertion indicator.
'litres' becomes 'L' (abbreviation).
'cause' enclosing 'l' is 'CLAUSE'.
'has' acts as a link.
(Other definitions for clause that I've seen before include "Single part in contract or treaty" , "Bill's article" , "Part of a sentence with its own finite verb" , "Subdivision of a sentence" , "Article in a contract" .)