Worn out - gruelling start is inappropriate (5)
I believe the answer is:
wrong
'inappropriate' is the definition.
(synonyms)
'worn out gruelling start' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'start' says to take the initial letters.
The initial letter of 'gruelling' is 'g'.
'worn' is an anagram of 'wron'.
'wron'+'g'='WRONG'
'is' acts as a link.
(Other definitions for wrong that I've seen before include "Unjust act" , "Unjust - incorrect" , "Injury - incorrect" , "bad" , "Treat unjustly" .)