Betray sources in the rank and file (5,5)
I believe the answer is:
grass roots
'the rank and file' is the definition.
(I've seen this before)
'betray sources' is the wordplay.
'betray' becomes 'grass' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sources' becomes 'roots' ('root' can be a synonym of 'source').
'grass'+'roots'='GRASS-ROOTS'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for grass roots that I've seen before include "Most basic level" , "Rank and file at the local level" , "gardening issue?" , "ordinary folk" , "Ordinary people (basis of lawn?)" .)