Firmly (4)
'firmly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for solidly that I've seen before include "Reliably, firmly" , "with substance" , "Working in a financially sound way" .)
'firmly' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for fast that I've seen before include "Go without food" , "Flying" , "requirement before surgery" , "Firmly attached; rapid" , "Quick - stop eating" .)