Saint sick in a boat? (7)
I believe the answer is:
sailing
'a boat?' is the definition.
(sailing boat is a kind of sailing vessel)
'saint sick' is the wordplay.
'saint' becomes 's'.
'sick' becomes 'ailing' ('ailing' can be similar in meaning to 'sick').
's'+'ailing'='SAILING'
'in' is the link.
(Other definitions for sailing that I've seen before include "Water sport" , "Going by, eg, yacht" , "Marine sport" , "event" , "Eg, yachting" .)