Some hero yearned to return long ago (4)
I believe the answer is:
yore
'long ago' is the definition.
(days of yore)
'some hero yearned to return' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'to return' says the letters should be written in reverse.
'eroy' can be found hidden inside 'hero yearned'.
'eroy' backwards is 'YORE'.
(Other definitions for yore that I've seen before include "Formerly, of long ago" , "Time long ago" , "Old usage for formerly, as in days of. . ." , "Bygone time" , "Long ago (poetic)" .)