Snake starts to emerge, checking the air (6)
I believe the answer is:
aspect
'air' is the definition.
(thesaurus)
'snake starts to emerge checking the' is the wordplay.
'snake' becomes 'asp' (asp is a kind of snake).
'starts to' says to take the initial letters.
The initial letters of 'emerge checking the' is 'ect'.
'asp'+'ect'='ASPECT'
(Other definitions for aspect that I've seen before include "The appearance presented" , "Viewpoint, feature to be considered" , "View or facet" , "'Appearance, outlook (6)'" , "The way something looks" .)