Rolls-Royce gradually develop internal gun (8)
I believe the answer is:
revolver
'gun' is the definition.
(revolver is a kind of gun)
'rolls-royce gradually develop internal' is the wordplay.
I cannot really see how this works, but
'develop' could be 'evolve' (synonyms) and 'evolve' is found in the answer.
'rolls' could be 'rr' (Rolls Royce) and 'rr' is found within the remaining letters.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for revolver that I've seen before include "Type of pistol" , "Six-shooter" , "needed for a roulette game?" , "Weapon holding maybe six rounds" , "murder weapon" .)