Benefit expert at one's disposal (9)

I believe the answer is:

available

'at one's disposal' is the definition.

'benefit expert' is the wordplay.
'benefit' becomes 'avail' (I've seen this before).
'expert' becomes 'able' (I've seen this in other clues).
'avail'+'able'='AVAILABLE'

(Other definitions for available that I've seen before include "Ready or able to be used" , "Not otherwise committed" , "Not otherwise occupied" , "Unoccupied" , "Obtainable or accessible" .)

I've seen this clue in The Telegraph.

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