Available officer ultimately in charge (4)
I believe the answer is:
free
'available' is the definition.
('free' can be similar in meaning to 'available')
'officer ultimately in charge' is the wordplay.
'ultimately' says to take the final letters.
'in' means one lot of letters goes inside another.
'charge' becomes 'fee' (fee is a kind of charge**).
The last letter of 'officer' is 'r'.
'r' going into 'fee' is 'FREE'.
(Other definitions for free that I've seen before include "Deliver" , "Not confined" , "Rescue" , "Not incurring charge" , "Not restricted or controlled" .)