Reason to include a number in a proviso (6)
I believe the answer is:
clause
'a proviso' is the definition.
(I've seen this in another clue)
'reason to include a number' is the wordplay.
'reason' becomes 'cause' (I've seen this before).
'to include' means one lot of letters goes inside another.
'a number' becomes 'L' (fifty in Roman numerals).
'cause' placed around 'l' is 'CLAUSE'.
'in' is the link.
(Other definitions for clause that I've seen before include "Bill's article" , "Joker, say" , "Subunit of a sentence" , "Section in a contract" , "More than a phrase and not quite a sentence" .)