Crime issue on unprotected barn (5)
I believe the answer is:
arson
'crime' is the definition.
(I've seen this before)
'issue on unprotected barn' is the wordplay.
'issue' becomes 'son' (issue can mean someone's offspring).
'on' means one lot of letters go next to another.
'unprotected' indicates the central letters.
The central letters of 'barn' are 'ar'.
'son' after 'ar' is 'ARSON'.
(Other definitions for arson that I've seen before include "Roans (anag)" , "serious offence" , "Fire-lighting crime" , "Act of a pyromaniac" , "Sonar (anag.)" .)