Engineer rings to fasten on the wagon (4)
I believe the answer is:
otto
'engineer rings' is the definition.
The definition suggests a singular noun which matches the answer.
'fasten on the wagon' is the wordplay.
I cannot quite understand how this works, but
'on' could be 'to' and 'to' is found within the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
A single letter 'o' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for otto that I've seen before include "Rose petal oil; Bismarck's name" , "Preminger or von Bismark?" , "Producer of engines" , "Emperor's" , "he goes" .)