Undiplomatic time to reduce activity (8)
I believe the answer is:
tactless
'undiplomatic' is the definition.
(synonyms)
'time to reduce activity' is the wordplay.
'time' becomes 't' (abbreviation).
'to reduce activity' becomes 'actless' (to act less is to reduce one's activity).
't'+'actless'='TACTLESS'
(Other definitions for tactless that I've seen before include "Lacking skill in handling delicate situations" , "Lacking diplomacy" , "Lacking sensitivity in dealing with others" , "Undiplomatic" , "Maladroit" .)