Fictional Captain seeing augury ascending (4)
I believe the answer is:
nemo
'fictional captain' is the definition.
(I've seen this in another clue)
'augury ascending' is the wordplay.
'augury' becomes 'omen' (omen is a kind of augury).
'ascending' says the letters should be written backwards.
'omen' in reverse letter order is 'NEMO'.
'seeing' acts as a link.
(Other definitions for nemo that I've seen before include "Finding -, animated film" , "Captain brought to book" , "- me impune lacessit (motto of Scotland)" , "Literary captain" , "Captain of the Nautilus" .)