Quick, put intense letters back inside shiny case (6)
I believe the answer is:
speedy
'quick' is the definition.
(I know that quick can be written as speedy)
'intense letters back inside shiny case' is the wordplay.
'intense letters back' becomes 'peed' (I can't justify this - if you can you should believe this answer much more).
'inside' indicates putting letters inside.
'case' means to remove the middle letters.
'shiny' with its middle removed is 'sy'.
'peed' going into 'sy' is 'SPEEDY'.
'put' is the link.
Can you help me to learn more?
(Other definitions for speedy that I've seen before include "Fast-moving" , "Moving quickly" , "Quick" , "Fast moving" , "in a hurry" .)