### Clue: Tense, mischievous elf crept up (10)
### Answer: PLUPERFECT 
 ### Explanation: Definition: Tense
  **Anagram of**(mischievous) ELF CREPT UP. 
Answer: In grammar, the tense that describes a verb whose action occurred even before the main verb that is itself in the past tense, eg. "*had gone* before I came".  Also known by the simpler "past perfect tense".