Yearn for sleep? About time (4)
I believe the answer is:
pant
'yearn' is the definition.
(thesaurus)
'sleep? about time' is the wordplay.
'sleep?' becomes 'nap' (napping is a kind of sleeping).
'about' says the letters should be written in reverse.
'time' becomes 't'.
'nap' back-to-front is 'pan'.
'pan'+'t'='PANT'
'for' acts as a link.
(Other definitions for pant that I've seen before include "Breathe hard" , "Breath rapidly" , "Breathe heavily" , "Sound out of breath" , "Gasp for breath (like a hot dog?)" .)