Use an online crossword solver to check possible answers. Read the clue carefully and break it down to understand all the wordplay. Look up unfamiliar words in a dictionary to learn their definitions. Search on the web for relevant topics that may relate to the clue.
Ask a friend if they recognize the clue or know the answer. Don’t give up – let the clue marinate and come back to it later with fresh eyes. If you’re still stuck, accept that it happens to all solvers and move on to another clue.
With careful reading and research, most clues can be cracked with a bit of persistence.