Going by boat, seamen initially unwell (7)
I believe the answer is:
sailing
'going by boat' is the definition.
'sailing' can be an answer for 'boat' (sailing boat is a kind of sailing vessel). I am not certain of the 'going by' bit.
'seamen initially unwell' is the wordplay.
'initially' says to take the initial letters.
'unwell' becomes 'ailing' (synonyms).
The first letter of 'seamen' is 's'.
's'+'ailing'='SAILING'
Can you help me to learn more?
(Other definitions for sailing that I've seen before include "Eg, yachting" , "(Of ship) leaving port" , "Water sport" , "Marine sport" , "Going by, eg, yacht" .)