The first son became unwell having gone by boat (6)
I believe the answer is:
sailed
'the first son became unwell having gone by boat' is the definition.
I can't tell whether this definition defines the answer.
'the first son became unwell having gone' is the wordplay.
'the first son' becomes 's' (ast letter of 'son').
'became unwell' becomes 'ai' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'having' means one lot of letters go next to another.
'gone' becomes 'led' ('lead' can be a synonym of 'go').
's'+'ai'+'led'='SAILED'
Can you help me to learn more?
(Other definitions for sailed that I've seen before include "Travelled by wind over water" , "Travelled by boat" .)