Committed to substitute position indeed! (7)
I believe the answer is:
deputed
'committed to substitute position indeed' is the definition.
I can't tell whether this definition defines the answer.
'committed to substitute position' is the wordplay.
'committed to' indicates putting letters inside (to commit can mean to imprison).
'substitute' becomes 'deed' (I can't explain this - if you can you should give a lot more credence to this answer).
'position' becomes 'put' (synonyms).
'deed' going around 'put' is 'DEPUTED'.
Can you help me to learn more?
(Other definitions for deputed that I've seen before include "Made a representative" , "Delegated" , "Assigned" .)