Back up thick-headedly, say? (3)
I believe the answer is:
put
'say?' is the definition.
(thesaurus)
'back up thick-headedly' is the wordplay.
'back' shows that the letters should be reversed in order.
'thick-headedly' becomes 't' (this might be a standard abbreviation I've not previously seen).
'up' in reverse letter order is 'pu'.
'pu'+'t'='PUT'
Can you help me to learn more?
(Other definitions for put that I've seen before include "Place in a position" , "Place; placed" , "Set (in place)" , "Place, lay" , "Place in a location" .)