clue vs fill in

clue

verb
  • To provide someone with information which he or she lacks (often used with "in" or "up"). 

  • To provide with a clue. 

noun
  • Information which may lead one to a certain point or conclusion. 

  • An object or a kind of indication which may be used as evidence. 

  • Insight or understanding ("to have a clue [about]" or "to have clue". See have a clue, clue stick) 

fill in

verb
  • To inform somebody, especially to supply someone missing or missed information. 

  • To substitute for somebody or something. 

  • To complete a form or questionnaire with requested information. 

  • To beat up; to physically assault. 

  • To fill; to replace material that is absent or has been removed. 

How often have the words clue and fill in occurred in a corpus of books? (source: Google Ngram Viewer )