Understand a source to make progress (3,5)
I believe the answer is:
get ahead
'make progress' is the definition.
(I've seen this before)
'understand a source' is the wordplay.
'understand' becomes 'get' (getting is a kind of understanding).
'source' becomes 'head' (head is a kind of source**).
'get'+'a'+'head'='GET AHEAD'
'to' acts as a link.
(Other definitions for get ahead that I've seen before include "Steal a march" , "The adage (anag) - make progress" , "At a hedge (anag) -- make progress" , "Make progress as in career, or take the lead" , "advance" .)