Arthur tries to conceal injury (4)
I believe the answer is:
hurt
'injury' is the definition.
(synonyms)
'arthur tries to conceal' is the wordplay.
'to conceal' indicates a hidden word.
'HURT' is hidden within 'arthur tries'.
(Other definitions for hurt that I've seen before include "John -, actor (The Elephant Man); harm" , "Cause pain to" , "Upset" , "Injure - injury" , "Feel pain" .)