Information about mid-Welsh valley (4)
I believe the answer is:
glen
'valley' is the definition.
(glen is a kind of valley)
'information about mid-welsh' is the wordplay.
'information' becomes 'gen' (gen is a kind of information).
'about' means one lot of letters goes inside another.
'mid' indicates the central letters.
The central letter of 'welsh' is 'l'.
'gen' enclosing 'l' is 'GLEN'.
(Other definitions for glen that I've seen before include "Valley, dale" , "Narrow, deep mountain valley" , "Campbell perhaps" , "Scottish mountain valley" , "Narrow Scots valley" .)