Facts involving large valley (4)
I believe the answer is:
glen
'valley' is the definition.
(I know that glen is a type of valley)
'facts involving large' is the wordplay.
'facts' becomes 'gen'.
'involving' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'gen' placed around 'l' is 'GLEN'.
(Other definitions for glen that I've seen before include "Narrow secluded valley in mountains" , "Campbell perhaps" , "Narrow Scots valley" , "Long narrow valley in Scotland" , "Depression in Scotland" .)