Second overdose in vein (4)
I believe the answer is:
mood
'vein' is the definition.
(thesaurus)
'second overdose' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'overdose' becomes 'od' ('o.d.' can be a synonym of 'overdose').
'mo'+'od'='MOOD'
'in' acts as a link.
(Other definitions for mood that I've seen before include "State of mind or emotion" , "melancholy" , "Humour - bad temper" , "Low spirits" , "Frame of mind" .)