A sign I left on somehow (4)
I believe the answer is:
lion
'a sign' is the definition.
(star sign Leo the Lion)
'i left on somehow' is the wordplay.
'left' becomes 'l' (common abbreviation).
'somehow' is an anagram indicator.
'i'+'l'+'on'='ilon'
'ilon' with letters rearranged gives 'LION'.
(Other definitions for lion that I've seen before include "Carnivorous 6" , "Touring rugby player" , "Important figure" , "cowardly character in Oz" , "Maned beast" .)