Applying some humus in garden (5)
I believe the answer is:
using
'applying' is the definition.
('use' can be a synonym of 'apply')
'some humus in garden' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'USING' can be found hidden inside 'humus in garden'.
(Other definitions for using that I've seen before include "Exploiting for one's own advantage" , "Putting to a particular purpose" , "Exercising" , "Employing" , "taking drugs" .)