Strips back one head's lethargy (10)
I believe the answer is:
sleepiness
'lethargy' is the definition.
(thesaurus)
'strips back one head's' is the wordplay.
'strips' becomes 'peels' (peeling is a kind of stripping).
'back' says the letters should be written backwards.
'one' becomes 'i' (Roman numeral).
'head' becomes 'ness' (ness is a headland).
'peels' backwards is 'sleep'.
'sleep'+'i'+'ness'='SLEEPINESS'
(Other definitions for sleepiness that I've seen before include "torpor" , "dozy quality" .)