Rower in doubt replaced boat driver (8)
I believe the answer is:
outboard
'boat driver' is the definition.
'outboard' can be an answer for 'boat' (thesaurus). I'm unsure of the 'driver' bit.
'rower in doubt replaced' is the wordplay.
'rower' becomes 'oar' (I've seen this before).
'in' means one lot of letters goes inside another.
'replaced' indicates anagramming the letters.
'doubt' with letters rearranged gives 'outbd'.
'oar' going within 'outbd' is 'OUTBOARD'.
Can you help me to learn more?
(Other definitions for outboard that I've seen before include "Engine external to the boat" , "Small boat's motor" , "Sort of motor" , "Detachable small boat motor" , "Type of boat engine" .)