Reason to include number in a proviso (6)
I believe the answer is:
clause
'a proviso' is the definition.
(I've seen this before)
'reason to include number' is the wordplay.
'reason' becomes 'cause' (I've seen this before).
'to include' means one lot of letters goes inside another.
'number' becomes 'l' (L is an example).
'cause' enclosing 'l' is 'CLAUSE'.
'in' is the link.
(Other definitions for clause that I've seen before include "Section in a contract" , "Subordinate part of sentence" , "More than a phrase and not quite a sentence" , "Subunit of a sentence" , "Verb-carrying component of sentence" .)