Go too far having completed project (9)
I believe the answer is:
overshoot
'go too far' is the definition.
(I've seen this before)
'completed project' is the wordplay.
'completed' becomes 'over' ('over' can be a synonym of 'completed').
'project' becomes 'shoot' (shooting is a kind of projecting**).
'over'+'shoot'='OVERSHOOT'
'having' is the link.
(Other definitions for overshoot that I've seen before include "Go beyond the mark" , "fail to stop where intended" , "Go extra distance" , "Go past unintentionally" , "Taxi too much, perhaps" .)