Came together after reserve went down (9)
I believe the answer is:
submerged
'went down' is the definition.
(submerging is a kind of going down)
'came together after reserve' is the wordplay.
'came together' becomes 'merged' (I've seen this in another clue).
'after' means one lot of letters go next to another.
'reserve' becomes 'sub' (substitute).
'merged' put after 'sub' is 'SUBMERGED'.
(Other definitions for submerged that I've seen before include "Completely covered by liquid" , "Under water" , "overwhelmed" .)