It determines who's next in command (5)
I believe the answer is:
order
'command' is the definition.
(I know that order is a type of command)
'it determines who's next' is the wordplay.
I cannot quite see how this works, but
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found within the answer.
'who' could be 'dr' (from TV series Doctor Who) and 'dr' is present in the remaining letters.
The remaining letters 'or' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for order that I've seen before include "rule of law" , "goods supplied" , "Call" , "Class - instruction - book" , "Jesuits, say" .)