Proficient in a small department (5)

I believe the answer is:

adept

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'proficient in' is the definition.
(thesaurus)

'a small department' is the wordplay.
'small department' becomes 'dept' (I am not sure about the 'small' bit.).
'a'+'dept'='ADEPT'

Can you help me to learn more?

(Other definitions for adept that I've seen before include "Having the required skills" , "capable" , "Thoroughly proficient" , "Accomplished" , "Practised" .)

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