Clarity is reversed by politician authorised on the fourth of July (10)
I believe the answer is:
simplicity
'clarity' is the definition.
(I've seen this before)
'is reversed by politician authorised on the fourth of july' is the wordplay.
'reversed' shows that the letters should be reversed in order.
'by politician' becomes 'mp' (Member of Parliament. I am not sure about the 'by' bit.).
'authorised' becomes 'licit' (licit can mean authorised or allowed).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'the fourth of july' becomes 'y' (fourth letter of the word 'July').
'is' backwards is 'si'.
'si'+'mp'+'licit'+'y'='SIMPLICITY'
Can you help me to learn more?
(Other definitions for simplicity that I've seen before include "Couldn't be easier!" , "Clarity" , "Facility, easiness" , "Ease" , "Lack of affectation or pretence" .)