Wrong sum, journalist thought (5)
I believe the answer is:
mused
'thought' is the definition.
(musing is a kind of thinking)
'wrong sum journalist' is the wordplay.
'wrong' indicates anagramming the letters.
'journalist' becomes 'ed' (short for editor).
'sum' is an anagram of 'mus'.
'mus'+'ed'='MUSED'
(Other definitions for mused that I've seen before include "Reflected or pondered" , "Was absorbed in thought" , "Was deep in thought" , "'Meditated, mulled (5)'" , "Closer" .)