PI played by Selleck (6)
I believe the answer is:
magnum
'pi played by selleck' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for magnum that I've seen before include "Arm" , "Large bottle of champagne, twice normal size" , "Double-size wine bottle" , "Just one bottle of champagne?" , "Twice the normal-size wine bottle" .)