Search round old valley (5)
I believe the answer is:
coomb
'valley' is the definition.
(I know that coomb is a type of valley)
'search round old' is the wordplay.
'search' becomes 'comb' (combing is a kind of searching).
'round' is an insertion indicator.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'comb' placed around 'o' is 'COOMB'.
(Other definitions for coomb that I've seen before include "Short valley or hollow" , "Hillside valley or hollow" , "Hollow on hill" .)