Professional's sin: 'blue skies' condemned (12)
I believe the answer is:
businesslike
'professional's' is the definition.
(I've seen this before)
'sin blue skies condemned' is the wordplay.
'condemned' indicates anagramming the letters.
'sin'+'blue'+'skies'='sinblueskies'
'sinblueskies' anagrammed gives 'BUSINESSLIKE'.
(Other definitions for businesslike that I've seen before include "Professional" , "Efficient" .)