Arbitrary rule again becomes real pain (9)
I believe the answer is:
neuralgia
'real pain' is the definition.
'arbitrary rule again' is the wordplay.
'arbitrary' indicates anagramming the letters (the original letters jumbled up).
'rule'+'again'='ruleagain'
'ruleagain' with letters rearranged gives 'NEURALGIA'.
'becomes' is the link.
(Other definitions for neuralgia that I've seen before include "illness" , "Severe pain through damaged nerves" , "Pain from damaged nerve" , "Source of pain" , "Acute pain of the nerves" .)