Wrong sum, hack thought (5)
I believe the answer is:
mused
'thought' is the definition.
(musing is a kind of thinking)
'wrong sum hack' is the wordplay.
'wrong' indicates anagramming the letters.
'hack' becomes 'ed' (hack can mean journalist, ed short for editor).
'sum' anagrammed gives 'mus'.
'mus'+'ed'='MUSED'
(Other definitions for mused that I've seen before include "Reflected in silence" , "Contemplated" , "Was deep in thought" , "Closer" , "Was absorbed in thought" .)