Reduce value of some land after dyke is emptied (7)
I believe the answer is:
detract
'reduce value' is the definition.
(I've seen this before)
'some land after dyke is emptied' is the wordplay.
'some land' becomes 'tract' (tract can mean an area of land).
'after' is a charade indicator (letters next to each other).
'is emptied' says to hollow out the word (remove centre letters).
'dyke' with its centre taken out is 'de'.
'tract' put after 'de' is 'DETRACT'.
'of' acts as a link.
(Other definitions for detract that I've seen before include "Devaluate" , "Diminish the worth (of)" , "Take away from, diminish" , "Abate" , "Reduce value" .)