Parties regularly supporting retrospective gun control (8)
I believe the answer is:
regulate
'control' is the definition.
(I've seen this before)
'parties regularly supporting retrospective gun' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'supporting' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'retrospective' shows that the letters should be reversed in order.
'gun' becomes 'luger' (type of pistol).
The alternate letters of 'parties' are 'ate'.
'luger' in reverse letter order is 'regul'.
'ate' put after 'regul' is 'REGULATE'.
(Other definitions for regulate that I've seen before include "Adjust - control" , "ADjust, fix" , "Ensure conformity with the rules" , "Police" , "exercise control" .)