Strong feeling of tiredness (3)
I believe the answer is:
ire
'strong feeling' is the definition.
(I've seen this before)
'of tiredness' is the wordplay.
'of' indicates a hidden word.
'IRE' is hidden in the letters of 'tiredness'.
(Other definitions for ire that I've seen before include "Strong feeling" , "Choler" , "Anger among admirers" , "Anger, rage" , "heat" .)