Sit up? Up indeed? It's sent as a substitute (9)
I believe the answer is:
deputised
'substitute' is the definition.
The answer and definition are different parts of speech. However, past participle verbs and adjectives occasionally mean the same thing.
'sit up? up indeed? it's sent as a' is the wordplay.
I cannot quite see how this works, but
an anagram of 'sit' is 'tis' which is within the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for deputised that I've seen is " Stood in (for)".)