In earnest as many an explorer (6)
I believe the answer is:
tasman
'explorer' is the definition.
(17th-century explorer Abel Tasman)
'in earnest as many an' is the wordplay.
'in' indicates a hidden word.
'TASMAN' is hidden in the letters of 'earnest as many an'.
(Other definitions for tasman that I've seen before include "Dutch antipodean explorer" , "Sea named after Dutch navigator" , "Abel ------, Dutch discoverer of New Zealand" , "Abel --, Dutch navigator" , "Australian strait" .)