Let rascal cry out 'it's obvious (7-5)
I believe the answer is:
crystal-clear
''it's obvious' is the definition.
(I've seen this before)
'let rascal cry out' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'let'+'rascal'+'cry'='letrascalcry'
'letrascalcry' is an anagram of 'CRYSTAL-CLEAR'.
(Other definitions for crystal-clear that I've seen before include "Completely transparent" , "In need of no further explanation" , "Absolutely evident" , "Easy to understand" , "Pellucid" .)