Described the steering apparatus in action (7)
I believe the answer is:
defined
'described the steering' is the definition.
The answer and definition can be both to do with communicating as well as being past participle verbs.
Maybe there's an association between them I don't understand?
'apparatus in action' is the wordplay.
'apparatus' becomes 'fin' (I can't explain this - if you can you should believe this answer much more).
'in' is an insertion indicator.
'action' becomes 'deed' (deed is a kind of action).
'fin' put within 'deed' is 'DEFINED'.
Can you help me to learn more?
(Other definitions for defined that I've seen before include "Gave the exact meaning of, say a word" , "Gave exact meaning or explained scope" .)