At first sight, a campfire's about to engulf one (5,5)
I believe the answer is:
prima facie
'at first sight' is the definition.
(synonyms)
'a campfire's about to engulf one' is the wordplay.
'about' indicates anagramming the letters.
'to engulf' indicates putting letters inside.
'one' becomes 'I' (Roman numeral).
'a'+'campfire'='acampfire'
'acampfire' anagrammed gives 'prmafacie'.
'prmafacie' placed around 'i' is 'PRIMA FACIE'.
(Other definitions for prima facie that I've seen before include "On first inspection" , "At first sight (Latin)" , "before detailed examination" , "usable evidence?" .)