Dashing knight mounted horse, holding hawk occasionally (6)
I believe the answer is:
rakish
'dashing' is the definition.
(I know that dashing can be written as rakish)
'knight mounted horse holding hawk occasionally' is the wordplay.
'knight' becomes 'sir' (a knight has the title Sir).
'mounted' says the letters should be written backwards (in a down clue, letters mounted up).
'horse' becomes 'H' (abbreviation as in horse power).
'holding' indicates putting letters inside.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
The alternating letters of 'hawk' are 'ak'.
'sir' in reverse letter order is 'ris'.
'ris'+'h'='rish'
'rish' going around 'ak' is 'RAKISH'.
(Other definitions for rakish that I've seen before include "Marked by unconventionality or disreputableness" , "Given to philandering" , "abandoned" , "Profligate" , "Dapper" .)