This clue uses a combination of techniques: taking a final letter from one word and placing it inside an anagram. Look for an anagram indicator and a word suggesting 'final' or 'last'.
'Finally' tells you to take the last letter of 'enrol' (which is L). 'Crash' is the anagram indicator - rearrange the letters of 'diet' and put the L inside this anagram.
Answer: TILDE | Take the final letter of 'enrol' (L) and put it inside an anagram of 'diet' (indicated by 'crash'). DIET anagrammed gives TIED, and L goes inside to make TILDE - the accent mark that appears over the ñ in 'mañana' but not in 'tomorrow'.