Searching on the way across Belgium (7)
'searching' is the definition.
(combing is a kind of searching)
'on the way across belgium' is the wordplay.
'on the way' becomes 'coming' (synonyms).
'across' indicates putting letters inside.
'belgium' becomes 'b' (international vehicle registration code).
'coming' going around 'b' is 'COMBING'.
(Other definitions for combing that I've seen before include "Systematically examining", "groom's role?".)