Sailor showing snake, beginning of nasty atmosphere aboard (7)
I believe the answer is:
boatman
'sailor' is the definition.
(I've seen this before)
'snake beginning of nasty atmosphere aboard' is the wordplay.
'snake' becomes 'boa' (boa constrictor, type of snake).
'beginning of' says to take the initial letters.
'atmosphere' becomes 'atm' (synonyms).
'aboard' is an insertion indicator.
The first letter of 'nasty' is 'n'.
'boa'+'n'='boan'
'boan' placed around 'atm' is 'BOATMAN'.
'showing' acts as a link.
(Other definitions for boatman that I've seen before include "One in charge of a small vessel" , "One having row with" , "Sailor" , "Water transport provider" , "Fellow with craft" .)