(n) Any word, often invented, written into a crossword solely because it fits with the known letters. The hamtrap is most frequently used when only one or two answers are missing, to give the 'solver' a sense of closure.
(v) To write such a word into a crossword
(n) "Did you put in 'chugfunt' as a hamtrap for 8 across? It's clearly meant to be 'clubfoot'."
(v) He couldn't get the last couple of answers in the crossword, so he just hamtrapped it.