A match of the greatest interest (4)
I believe the answer is:
test
'a match' is the definition.
(test match is a kind of match)
'of the greatest interest' is the wordplay.
'of' indicates a hidden word.
'TEST' is hidden within 'greatest interest'.
(Other definitions for test that I've seen before include "Cricket match; try" , "Critical examination or trial of qualities or abilities" , "Trial - international game" , "big game" , "Trial; Hampshire river" .)