Son dividing silver isn’t commonly in opposition (7)
I believe the answer is:
against
'in opposition' is the definition.
(I've seen this before)
'son dividing silver isn't commonly' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'dividing' means one lot of letters goes inside another (inserted letters divide the word).
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'isn't commonly' becomes 'ain't' (non-standard version of 'isn't').
'ag'+'aint'='againt'
's' inserted inside 'againt' is 'AGAINST'.
(Other definitions for against that I've seen before include "Con" , "Anti, touching" , "Not supporting" , "In disagreement with" , "on the other side" .)