First-rate dispute drained adviser (4)
I believe the answer is:
aide
'adviser' is the definition.
'first-rate dispute drained' is the wordplay.
'first-rate' becomes 'ai' (resembles A1, informal term for excellent).
'drained' says to hollow out the word (remove centre letters).
'dispute' with its centre removed is 'de'.
'ai'+'de'='AIDE'
(Other definitions for aide that I've seen before include "Officer who acts as military assistant" , "auxiliary" , "Helper" , "a help" , "Personal assistant in the military" .)