A blow afloat? (5)
I believe the answer is:
smack
'a blow' is the definition.
(both can mean a hit)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for smack that I've seen before include "Sharp blow; sailing boat" , "Slap the fishing vessel" , "Hit - kiss - fishing boat - heroin" , "Heroin (sl.)" , "Suggestion" .)