Try an overdue alternative (9)
I believe the answer is:
endeavour
'try' is the definition.
(synonyms)
'an overdue alternative' is the wordplay.
'alternative' indicates an anagram.
'an'+'overdue'='anoverdue'
'anoverdue' with letters rearranged gives 'ENDEAVOUR'.
(Other definitions for endeavour that I've seen before include "Strive - Inspector Morse's first name" , "Try to achieve something" , "Enterprise; earnest effort" , "Aspire; attempt" , "Make a serious effort" .)