This uses a combination of deletion and charade. Deletion removes letters from a word (often the middle or ends), and charade joins word parts together like building blocks.
'Vacant' indicates that the middle letters should be removed from 'colonel', leaving just the outer letters. Then add a word meaning 'temper' to complete the answer.
Answer: CLANGER | 'Vacant colonel' means remove the middle letters from COLONEL, leaving CL. Add ANGER (temper) to get CL + ANGER = CLANGER (stupid mistake).