Motorless, they’re surprisingly fast on the river (5)
I believe the answer is:
rafts
'motorless' is the definition.
The answer and definition are not the same part of speech.
'surprisingly fast on the river' is the wordplay.
'surprisingly' is an anagram indicator.
'on' is an insertion indicator (as in clothing \'on\' a person).
'the river' becomes 'r' (**).
'fast' with letters rearranged gives 'afts'.
'afts' going around 'r' is 'RAFTS'.
'they're' acts as a link.
Can you help me to learn more?
(Other definitions for rafts that I've seen before include "Flat boats" , "Flat floats" , "Flat floating structures" , "Flat, floating platforms" .)