Crossword Genius

A boat trip when things are going smoothly (4)

Ross

I believe the answer is:

sail

I'm a little stuck... Click here to teach me more about this clue!

'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" .)

I've seen this clue in the Evening Standard.
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