An ultra can be wild (7)
I believe the answer is:
natural
'wild' is the definition.
(synonyms)
'an ultra can be' is the wordplay.
'can be' is an anagram indicator.
'an'+'ultra'='anultra'
'anultra' anagrammed gives 'NATURAL'.
(Other definitions for natural that I've seen before include "Unforced" , "Unvarnished" , "Normal - innate - instinctive" , "Not artificial; off-white" , "Inborn; unaffected" .)