Hard setter on the Guardian ultimately supporting application (11)

'application' is the definition.
(eg the application of rules)

'hard setter on the guardian ultimately supporting' is the wordplay.
'hard setter' becomes 'cement' (cement is hard once it sets).
'on' is a charade indicator (letters next to each other).
'ultimately' says to take the final letters.
'supporting' becomes 'for' (being for something means supporting it).
The final letters of 'the guardian' are 'en'.
'en'+'for'='enfor'
'cement' after 'enfor' is 'ENFORCEMENT'.