Summary by Oscar piercing in exactitude (9)
I believe the answer is:
precision
'exactitude' is the definition.
(I know that precision is a type of exactitude)
'summary by oscar piercing in' is the wordplay.
'summary' becomes 'precis' (precis is a kind of summary).
'by' says to put letters next to each other.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'piercing' means one lot of letters goes inside another (some letters pierce a hole in a word).
'o' inserted into 'in' is 'ion'.
'precis'+'ion'='PRECISION'
(Other definitions for precision that I've seen before include "Care" , "Accuracy, exactness" , "Exactness, sharpness" , "Meticulousness" , "Exactitude" .)