Definition: "One chooses"

This uses a container clue - one set of letters is placed inside another set of letters that have been modified in some way.

Look for an abbreviation for 'shock treatment' that goes inside the letters of 'role' when they're reversed. The reversal indicator is 'reversal'.

Answer: ELECTOR | ECT (abbreviation for ElectroConvulsive Therapy, 'shock treatment') goes inside ELOR (reversal of 'role') = EL-ECT-OR