One getting on in charge of architectural style (5)
I believe the answer is:
ionic
'architectural style' is the definition.
(I've seen this before)
'one getting on in charge' is the wordplay.
'one' becomes 'i' (Roman numeral).
'getting' means one lot of letters go next to another.
'in charge' becomes 'ic'.
'i'+'on'+'ic'='IONIC'
'of' acts as a link.
(Other definitions for ionic that I've seen before include "Sort of column" , "Support (a weight)" , "Style of Greek column used for the British Museum and the US Capitol" , "Greek architectural style" , "Relating to charged particles" .)