Clear vacant land up and finally solve mystery (6)
I believe the answer is:
riddle
'mystery' is the definition.
(I've seen this before)
'clear vacant land up and finally solve' is the wordplay.
'clear' becomes 'rid' (synonyms).
'vacant' means to remove the middle letters (a vacant or emptied out form of the word).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'and' means one lot of letters go next to another.
'finally' indicates one should take the final letters.
'land' with its centre taken out is 'ld'.
The last letter of 'solve' is 'e'.
'ld' backwards is 'dl'.
'rid'+'dl'+'e'='RIDDLE'
(Other definitions for riddle that I've seen before include "Cryptic question" , "Cryptic verse" , "Brain-teaser and coarse sieve" , "unsolved problem" , "mystery" .)