Eli's back in before mother superior (7)
I believe the answer is:
premium
'superior' is the definition.
(thesaurus)
'eli's back in before mother' is the wordplay.
'back' says to take the final letters (the letter at the back).
'in' is an insertion indicator.
'before' becomes 'pre' (prefix meaning 'before').
'mother' becomes 'mum' (informal term for mother).
The final letter of 'eli' is 'i'.
'pre'+'mum'='premum'
'i' going into 'premum' is 'PREMIUM'.
(Other definitions for premium that I've seen before include "Extra cost of something in short supply" , "Superior; amount paid for insurance" , "bonus payment" , "Great value - bonus" , "cash needed for cover charge" .)