Accomplished head of admin department (5)
I believe the answer is:
adept
'accomplished' is the definition.
(synonyms)
'head of admin department' is the wordplay.
'head of' suggests taking the first letters.
'department' becomes 'dept' ().
The first letter of 'admin' is 'a'.
'a'+'dept'='ADEPT'
(Other definitions for adept that I've seen before include "Very skilful, proficient" , "Highly proficient" , "One very skilled at work" , "Thoroughly proficient" , "Skilled, expert" .)