Skilful prosecutor turned and went on scratching head (5)
I believe the answer is:
adept
'skilful' is the definition.
(synonyms)
'prosecutor turned and went on scratching head' is the wordplay.
'prosecutor' becomes 'D.A.' (District Attorney).
'turned' is a reversal indicator.
'and' is a charade indicator (letters next to each other).
'went on' becomes 'kept' ('keep' can be a synonym of 'go on').
'scratching head' means to remove the first letter (the word's head is removed).
'da' back-to-front is 'ad'.
'kept' with its initial letter taken off is 'ept'.
'ad'+'ept'='ADEPT'
(Other definitions for adept that I've seen before include "skilled performer" , "Having the required skills" , "Dexterous" , "clever" , "Proficient, skilled" .)