Exposed, a bull never relaxed (10)
I believe the answer is:
vulnerable
'exposed' is the definition.
(I've seen this before)
'a bull never relaxed' is the wordplay.
'relaxed' indicates anagramming the letters.
'a'+'bull'+'never'='abullnever'
'abullnever' with letters rearranged gives 'VULNERABLE'.
(Other definitions for vulnerable that I've seen before include "Exposed" , "Susceptible" .)