Unprotected, ultimately bitten by a sheep fly (5)
I believe the answer is:
naked
'unprotected' is the definition.
(similar in meaning)
'ultimately bitten by a sheep fly' is the wordplay.
'ultimately' indicates one should take the final letters.
'by' says to put letters next to each other.
'sheep fly' becomes 'ked' (fly living in the wool of sheep).
The final letter of 'bitten' is 'n'.
'n'+'a'+'ked'='NAKED'
(Other definitions for naked that I've seen before include "Starkers" , "Uncovered" , "In the altogether" , "Knead (anag.)" , "Unclad" .)