A reservation briefly claimed by very old rancher (7)
I believe the answer is:
vaquero
'rancher' is the definition.
(Spanish term for a cowboy or cattle-driver)
'a reservation briefly claimed by very old' is the wordplay.
'reservation' becomes 'query' (both can mean a point of doubt or confusion).
'briefly' means to remove the last letter.
'claimed by' is an insertion indicator.
'very' becomes 'v' (abbreviation).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'a'+'query'='aquery'
'aquery' with its last letter removed is 'aquer'.
'v'+'o'='vo'
'aquer' put inside 'vo' is 'VAQUERO'.
(Another definition for vaquero that I've seen is " Cowboy".)