Worry about going stern-first into a waterfall (4)
I believe the answer is:
fret
'worry' is the definition.
(to fret about something is to worry)
'about going stern-first into a waterfall' is the wordplay.
I cannot quite see how this works, but
'about' could be 're' ('re' means regarding or about) and 're' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fret that I've seen before include "Chafe; worry" , "Be anxious; ornament" , "Be anxious -- part of a fingerboard" , "Bar on fingerboard of guitar" , "Worry; sort of saw" .)