Four in five answer test (4)
I believe the answer is:
viva
'test' is the definition.
(I've seen this before)
'four in five answer' is the wordplay.
'four' becomes 'IV' (roman numeral).
'in' indicates putting letters inside.
'five' becomes 'V' (roman numeral).
'answer' becomes 'a' (abbreviation eg in Q and A).
'v'+'a'='va'
'iv' put inside 'va' is 'VIVA'.
(Other definitions for viva that I've seen before include "academic exam!" , "Long live! - an examination" , "Hail to the Spaniards" , "Oral examination" , "Kind of exam" .)