Drastically affect completed objective (5)
I believe the answer is:
upend
'drastically affect' is the definition.
(I've seen this before)
'completed objective' is the wordplay.
'completed' becomes 'up' (I am not sure about this - if you are sure you should believe this answer much more).
'objective' becomes 'end' (both can mean a purpose).
'up'+'end'='UPEND'
Can you help me to learn more?
(Other definitions for upend that I've seen before include "Turn upside-down" , "Turn upside down" , "Stand erect" , "Invert" , "Flip" .)