A boat trip when things are going smoothly (4)
I believe the answer is:
sail
'a boat' is the definition.
(I know that sail is a more specific form of boat)
'trip when things are going smoothly' is the wordplay.
I cannot quite understand how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found within the answer.
The remaining letters 'sil' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for sail that I've seen before include "Go by water" , "Canvas on board" , "Fabric on a mast" , "Begin a voyage" , "Travel by boat" .)