Improving soil with family material (7)
I believe the answer is:
oilskin
'material' is the definition.
(I've seen this before)
'improving soil with family' is the wordplay.
'improving' indicates an anagram (letters must be sorted out or rectified).
'with' means one lot of letters go next to another.
'family' becomes 'kin' (synonyms).
'soil' with letters rearranged gives 'oils'.
'oils'+'kin'='OILSKIN'
(Other definitions for oilskin that I've seen before include "Waterproof cloth for rain-gear" , "material" , "protective cloth" , "Waterproof fabric" , "security for a rainy day" .)