Bank making soil function (9)
I believe the answer is:
earthwork
'bank' is the definition.
(thesaurus)
'soil function' is the wordplay.
'soil' becomes 'earth' (synonyms).
'function' becomes 'work' (synonyms).
'earth'+'work'='EARTHWORK'
'making' acts as a link.
(Other definitions for earthwork that I've seen before include "Large artificial soil bank" , "Rampart made of soil" , "Banking" , "Bank made to defend" , "Defensive bank of soil" .)