Mean insulin to be used regularly for tiredness? (5)
I believe the answer is:
ennui
'tiredness?' is the definition.
(I've seen this before)
'mean insulin to be used regularly' is the wordplay.
'to be used regularly' indicates alternate letters.
'mean'+'insulin'='meaninsulin'
The alternate letters of 'meaninsulin' are 'ENNUI'.
'for' is the link.
(Other definitions for ennui that I've seen before include "Boredom, dullness" , "Mental weariness" , "to make one list less when connected" , "World-weariness" , "French boredom and lassitude" .)