Solid shell of turtle breaking down rapidly (9)
I believe the answer is:
steadfast
'solid' is the definition.
(thesaurus)
'shell of turtle breaking down rapidly' is the wordplay.
'shell of' means to remove the middle letters.
'breaking' indicates putting letters inside.
'down' becomes 'sad' (I've seen this before).
'rapidly' becomes 'fast' (synonyms).
'turtle' with its middle taken out is 'te'.
'te' inserted into 'sad' is 'stead'.
'stead'+'fast'='STEADFAST'
(Other definitions for steadfast that I've seen before include "Constant and resolute" , "Dependable, unwavering" , "Firm and dependable" , "Resolutely firm" , "Strong" .)