Father, say, with reason to include large part of sentence (8,6)
I believe the answer is:
relative clause
'sentence' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'father say with reason to include large' is the wordplay.
'father say' becomes 'relative' (father is a kind of relative).
'with' says to put letters next to each other.
'reason' becomes 'cause' (I've seen this before).
'to include' is an insertion indicator.
'large' becomes 'L' (eg in clothes sizes).
'cause' enclosing 'l' is 'clause'.
'relative'+'clause'='RELATIVE CLAUSE'
'part of' acts as a link.
Can you help me to learn more?
(Another definition for relative clause that I've seen is " who introduces one".)