Crime is a primarily rational issue (5)
I believe the answer is:
arson
'crime' is the definition.
(I've seen this before)
'a primarily rational issue' is the wordplay.
'primarily' says to take the initial letters.
'issue' becomes 'son' (issue can mean someone's offspring).
The first letter of 'rational' is 'r'.
'a'+'r'+'son'='ARSON'
'is' is the link.
(Other definitions for arson that I've seen before include "A burning crime" , "Roans (anag)" , "Incendiary act" , "Incendiary crime" , "Fire-raising crime" .)