Dashing like a fly, arguably, did you say? (5)
I believe the answer is:
natty
'dashing' is the definition.
('natty' can be a synonym of 'dashing')
I don't understand how the remainder of the clue works.
(Other definitions for natty that I've seen before include "Stylish" , "Smart, spruce" , "Smart, fashionable (colloq.)" , "Dapper (colloq.)" , "Smartly dressed" .)