Used to be tense, discontented? Rubbish! (5)
I believe the answer is:
waste
'rubbish' is the definition.
(thesaurus)
'used to be tense discontented?' is the wordplay.
'used to be' becomes 'was' (I've seen this before).
'discontented?' means to remove the middle letters.
'tense' with its middle taken out is 'te'.
'was'+'te'='WASTE'
(Other definitions for waste that I've seen before include "Use to no purpose" , "Spend thoughtlessly or use inefficiently" , "Squander; spoil" , "desert" , "Useless leftovers" .)