Rubbish can get head going off (7)
I believe the answer is:
rotting
'off' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?
'rubbish can get head going' is the wordplay.
'rubbish' becomes 'rot' (both can mean nonsense).
'can' becomes 'tin' (both are metal containers).
'get' says to put letters next to each other.
'head going' becomes 'g' (I am not sure about this - if you are sure you should believe this answer much more).
'rot'+'tin'+'g'='ROTTING'
Can you help me to learn more?
(Other definitions for rotting that I've seen before include "Putrid" , "Sultanate in NW Borneo" , "Going bad" , "corruption?" , "Decaying" .)