Demanding levy in force (6)
I believe the answer is:
taxing
'demanding' is the definition.
(thesaurus)
'levy in force' is the wordplay.
'levy' becomes 'tax' (tax is a kind of levy).
'force' becomes 'g'.
'tax'+'in'+'g'='TAXING'
(Other definitions for taxing that I've seen before include "Physically or mentally demanding" , "Burdensome" , "hard work" , "'Not easily borne, very wearing (6)'" , "Getting money off people" .)