Being on top of exceptionally good job (13)
I believe the answer is:
superposition
'being on top' is the definition.
(superposition is placing one thing over another)
'exceptionally good job' is the wordplay.
'exceptionally good' becomes 'super' (super can mean very good).
'job' becomes 'position' (position is a kind of job).
'super'+'position'='SUPERPOSITION'
'of' is the link.
(Other definitions for superposition that I've seen before include "Getting on top of things" , "The state of being above something" .)