Unusual pattern a ruler re-established (13)
I believe the answer is:
preternatural
'unusual' is the definition.
(I've seen this before)
'pattern a ruler re-established' is the wordplay.
're-established' is an anagram indicator.
'pattern'+'a'+'ruler'='patternaruler'
'patternaruler' is an anagram of 'PRETERNATURAL'.
(Other definitions for preternatural that I've seen before include "Abnormal" , "beyond what's normal" , "Weird - extraordinary" , "Very unusual" .)