One working, in charge of classic architecture (5)
I believe the answer is:
ionic
'classic architecture' is the definition.
(I've seen this in another clue)
'one working in charge' is the wordplay.
'one' becomes 'i' (Roman numeral).
'working' becomes 'on' (on can mean working or activated).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'i'+'on'+'ic'='IONIC'
'of' is the link.
(Other definitions for ionic that I've seen before include "Greek tongue" , "Support (a weight)" , "Like some Greek architecture" , "Style of Greek column used for the British Museum and the US Capitol" , "entailing small charges" .)