Thus working shortly (4)
I believe the answer is:
soon
'shortly' is the definition.
(synonyms)
'thus working' is the wordplay.
'thus' becomes 'so' (synonyms).
'working' becomes 'on' (on can mean working or activated).
'so'+'on'='SOON'
(Other definitions for soon that I've seen before include "Presently" , "In a little while" , "Before long" , "in a minute" , "Willingly" .)