Vessel going from side to side (5)
I believe the answer is:
'vessel' is the definition.
(ferry is a kind of vessel)
'going from side to side' is the wordplay.
I cannot quite see how this works, but
'from' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
'side' could be 'r' (abbreviation for right) and 'r' is present in the answer.
'to side' could be 'r' (abbreviation for right) and 'r' is present in the answer.
The remaining letters 'fy' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ferry that I've seen before include "Carry by boat" , "Passenger ship travelling regularly over a short distance" , "Boat that provides transport over stretch of water" , "Boat across e.g. English Channel" , "Transport boat that plies regularly" .)