Rash unionist, one originally taking part in drive (9)
I believe the answer is:
impetuous
'rash' is the definition.
(thesaurus)
'unionist one originally taking part in drive' is the wordplay.
'originally' indicates taking the first letters.
'taking part in' is an insertion indicator.
'drive' becomes 'impetus' (impetus is a kind of drive**).
The initial letters of 'unionist one' is 'uo'.
'uo' inserted into 'impetus' is 'IMPETUOUS'.
(Other definitions for impetuous that I've seen before include "Acting rashly" , "Impulsive, hasty" , "Rash" , "Hot-headed" .)