Lose years, dreadfully idle (5)
I believe the answer is:
yield
'lose' is the definition.
(thesaurus)
'years dreadfully idle' is the wordplay.
'years' becomes 'y' (abbreviation).
'dreadfully' is an anagram indicator (a dreadful spelling).
'idle' with letters rearranged gives 'ield'.
'y'+'ield'='YIELD'
(Other definitions for yield that I've seen before include "Render" , "Chicken out" , "Submit, give way" , "Amount produced" , "admit defeat" .)