Starting tomorrow, work in diplomacy (4)
I believe the answer is:
tact
'diplomacy' is the definition.
(tact can mean diplomacy or sensitivity)
'starting tomorrow work' is the wordplay.
'starting' suggests taking the first letters.
'work' becomes 'act' (synonyms).
The initial letter of 'tomorrow' is 't'.
't'+'act'='TACT'
'in' acts as a link.
(Other definitions for tact that I've seen before include "measure of diplomacy?" , "Consideration and sensitivity" , "Skill in dealing with people" , "Understanding" , "Delicacy of feeling, skill in saying the right thing" .)