Abnormal capers, astronomical length (6)
I believe the answer is:
parsec
'astronomical length' is the definition.
(I've seen this in another clue)
'abnormal capers' is the wordplay.
'abnormal' is an anagram indicator.
'capers' is an anagram of 'PARSEC'.
(Other definitions for parsec that I've seen before include "Long way" , "Three-and-a-bit light-years" , "3.26 light years" , "it takes years to get this far" , "far in the distance" .)