Weaken soil in land (4,4)
I believe the answer is:
wear thin
'weaken' is the definition.
(thesaurus)
'soil in land' is the wordplay.
'soil' becomes 'earth' (synonyms).
'in' is an insertion indicator.
'land' becomes 'win' (both can mean to secure).
'earth' placed within 'win' is 'WEAR THIN'.
(Other definitions for wear thin that I've seen before include "Begin to fail" , "Diminish - become less tolerable" , "Start to fail" , "Become threadbare" , "Become less convincing through overuse" .)