Grow together, fuse (8)
I believe the answer is:
coalesce
I believe this is a double definition.
'grow together' is the first definition.
(I've seen this in another clue)
'fuse' is the second definition.
(I know that fuse can be written as coalesce)
(Other definitions for coalesce that I've seen before include "Combine and form one whole, as political parties may" , "Combine with others as political parties may do" , "Grow together, amalgamate" , "To bring together" , "blend" .)