### Clue: Maintain senility is common (7)
### Answer: AVERAGE 
 ### Explanation: Definition: common
  AVER(assert,*maintain* a statement is true) AGE(advanced years,old age,*senility*.  As used here, "senility" merely means the state of old age, as distinct from the "infirmity" or "weakness" associated with old age.  A substitute word, more unambiguous, would have been "senescence".)