Very bad sailor capsized at first in stream (6)
I believe the answer is:
abject
'very bad' is the definition.
'sailor capsized at first in stream' is the wordplay.
'sailor' becomes 'ab' (abbreviation for able seaman).
'at first' indicates taking the first letters.
'in' means one lot of letters goes inside another.
'stream' becomes 'jet'.
The initial letter of 'capsized' is 'c'.
'c' placed within 'jet' is 'ject'.
'ab'+'ject'='ABJECT'
(Other definitions for abject that I've seen before include "Down-hearted" , "Mean, wretched" , "Lacking all pride, grovelling" , "Extremely humble" , "Of the most contemptible kind" .)