Joins or merges to form a single unit (8)
I believe the answer is:
combines
'joins or merges to form a single unit' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for combines that I've seen before include "Joins or puts together" , "Joins or merges to form a single unit" , "blends" , "'Joins together, unites (8)'" , "Put together" .)