The crossword solution for the term SUFFIX FOR SONNET is 'EER', which is a three-letter suffix. This suffix is commonly used to denote a poetic form, such as a sonnet. It is derived from the Middle English word 'er' meaning 'one who does something'. Thus, when added to the end of a word, it implies that the word is related to a particular activity or profession. For example, the suffix 'eer' can be added to the word 'POET' to create the word 'poeteer', which refers to someone who writes poetry.