Horse covering miles in search (4)
I believe the answer is:
comb
'search' is the definition.
(I know that comb is a more specific form of the action search)
'horse covering miles' is the wordplay.
'horse' becomes 'cob' (type of horse).
'covering' indicates putting letters inside.
'miles' becomes 'm' (common abbreviation).
'cob' placed around 'm' is 'COMB'.
'in' acts as a link.
(Other definitions for comb that I've seen before include "Ransack, scour" , "Tidy (hair); some honey" , "Groom; search" , "Crest on the head of some birds" , "Toothed device - crest" .)