Thick-headed, foolhardy riff-raff (5)
I believe the answer is:
trash
'raff' is the definition.
(thesaurus)
'thick-headed foolhardy' is the wordplay.
'thick-headed' becomes 't' (this might be a standard abbreviation I've not previously seen).
'foolhardy' becomes 'rash' (synonyms).
't'+'rash'='TRASH'
'riff' is the link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for trash that I've seen before include "Waste material (US)" , "Rubbish, refuse" , "bull" , "Rubbish, litter" , "Refuse; wreck" .)