Journey to a central source (4)
I believe the answer is:
tour
'journey' is the definition.
(touring is a kind of journeying)
'to a central source' is the wordplay.
'a central' means to look at the middle letters.
The centre of 'source' is 'ur'.
'to'+'ur'='TOUR'
(Other definitions for tour that I've seen before include "Spell of military duty" , "Extended journey" , "Go round" , "Drive" , "Term of duty" .)