Rule found by officer to stop airship heading off is not able to be changed (12)
I believe the answer is:
incorrigible
'rule found by officer to stop airship heading off is not able to be changed' is the definition.
I can't judge whether this definition defines the answer.
'to stop airship heading off is not able to be changed' is the wordplay.
'to stop' means one lot of letters goes inside another (to stop can mean to block a gap).
'airship' becomes 'dirigible' ('dirigible' can be a synonym of 'airship').
'heading off' means to remove the first letter.
'is not able to be changed' becomes 'ncor' (I can't explain this - if you can you should believe this answer much more).
'dirigible' with its initial letter removed is 'irigible'.
'irigible' placed around 'ncor' is 'INCORRIGIBLE'.
Can you help me to learn more?
(Other definitions for incorrigible that I've seen before include "Not reformable" , "Unable to be reformed" , "Beyond correction" , "beyond redemption" , "Firmly" .)