Weaken resistance of son frequently before beak (6,2)
I believe the answer is:
soften up
'weaken resistance of' is the definition.
(to soften up is to make more receptive)
'son frequently before beak' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'frequently' becomes 'often' (synonyms).
'before beak' becomes 'up' (up before a magistrate - beak is a slang term).
's'+'often'+'up'='SOFTEN UP'
(Other definitions for soften up that I've seen before include "Undermine the resistance (of someone)" , "Work on" .)