Witness reportedly ill on banks of Nile aboard ship with nausea caused by motion (11)
I believe the answer is:
seasickness
'banks of nile aboard ship with nausea caused by motion' is the definition.
I can't tell whether this defines the answer.
'witness reportedly ill' is the wordplay.
'witness' becomes 'see' (synonyms).
'reportedly' shows a homophone (sound like).
'ill' becomes 'sickness' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'see' is a homophone of 'sea'.
'sea'+'sickness'='SEASICKNESS'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for seasickness that I've seen before include "Mal de mer" , "Marine nausea" .)