Dedication of mounted police unit imprisoning drunk Nigel Farage at last (9)
I believe the answer is:
diligence
'dedication' is the definition.
(I've seen this before)
'mounted police unit imprisoning drunk nigel farage at last' is the wordplay.
'mounted' says the letters should be written in reverse (in a down clue, letters mounted up).
'police unit' becomes 'CID' (abbreviation for Criminal Investigation Department).
'imprisoning' indicates putting letters inside.
'drunk' is an anagram indicator.
'at last' suggests the final letters.
The last letter of 'farage' is 'e'.
'cid' written backwards gives 'dic'.
'nigel' anagrammed gives 'ligen'.
'dic' enclosing 'ligen' is 'diligenc'.
'diligenc'+'e'='DILIGENCE'
'of' is the link.
(Other definitions for diligence that I've seen before include "Careful and persistent work or effort" , "needing proper care and attention" , "Stage coach" , "Old stagecoach" , "It takes steady application" .)