One taking an active part grabbing pit boss (8)
I believe the answer is:
domineer
'boss' is the definition.
(I've seen this before)
'one taking an active part grabbing pit' is the wordplay.
'one taking an active part' becomes 'doer' (to do is to act).
'grabbing' means one lot of letters goes inside another.
'pit' becomes 'mine' (mining pit).
'doer' going around 'mine' is 'DOMINEER'.
(Other definitions for domineer that I've seen before include "behave arrogantly" , "Tyrrannise" , "Tyrannise" , "Browbeat" , "Ride roughshod over" .)