From Longman Dictionary of Contemporary Englishfill in phrasal verb1documentWRITE fill something ↔ in to write all the necessaryinformation on an officialdocument, form etcDon’t forget to fill in your boarding cards.2tell somebody news fill somebody ↔ inTELL to tell someone about recentevents, especially because they have been away from a placefill somebody ↔ in onI think you’d better fill me in on what’s been happening.3crack/holeREPAIR fill something ↔ in to put a substance into a hole, crack etc so it is completely full and level4SPEND TIME fill in time to spend time doing something unimportant because you are waiting for something to happenShe flipped through a magazine to fill in the time.5spaceDRAW fill something ↔ in to paint or draw over the spaceinside a shape6do somebody’s jobJOB/WORK to do someone’s job because they are not there forI’m filling in for Joe for a few days. →fill→ See Verb table
fill-inˈfill-in noun [singular] British English informalBEsomeone who does someone else’s job because that person is not there syn stand-inI’m here as a fill-in while Robert’s away.
Examples from the Corpus
fill-in• Also, make sure that you improvisevariations on the fill-ins at the end of each two barphrase.• Hahn is scheduled to announce tonight's game as a fill-in for Joe Starkey.• Royston-Cambridge and Watford-St Albans fill-inschemes were also approved.From Longman Business Dictionaryfill something → in phrasal verb [transitive]to write all the information that is requested on an official form SYN FILL OUTHe filled in the usual trader’s form offering to sell the car to the finance company.The time sheet is filled in by your supervisor. →fill→ See Verb tablefill-inˈfill-in noun [countable] informala person who does another person’s job for a short period of time, because the other person is not thereI was just a fill-in when she wasn’t available.