Alien investigator not unknown to row (5)
I believe the answer is:
scull
'to row' is the definition.
(to scull is to row a boat with sculls)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for scull that I've seen before include "Light rowing boat" , "Small oar" , "racing shell" , "Row with two oars" , "A single oar" .)