Activist, male one, left it with worker (8)
I believe the answer is:
militant
'activist' is the definition.
(I know that activist can be written as militant)
'male one left it with worker' is the wordplay.
'male' becomes 'm' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'left' becomes 'l' (common abbreviation).
'with' says to put letters next to each other.
'worker' becomes 'ant' (worker is a type of ant).
'm'+'i'+'l'+'it'+'ant'='MILITANT'
(Other definitions for militant that I've seen before include "In the mood to fight" , "Aggressive, willing to fight" , "Up in arms" , "Aggressive (tendency)" , "In mood for aggressive action" .)