Rapid speech involving decorative design mostly (6)
I believe the answer is:
patter
'rapid speech' is the definition.
(I've seen this in another clue)
'decorative design mostly' is the wordplay.
'decorative design' becomes 'pattern' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'pattern' with its final letter taken away is 'PATTER'.
'involving' is the link.
(Other definitions for patter that I've seen before include "Spiel" , "Noise of fast light steps" , "Light tapping sounds as of little feet" , "Salesman's chat - walk with quick, soft steps" , "Comedian's rapid jokes" .)