Quick little boy plunged in Scottish river (6)
I believe the answer is:
speedy
'quick' is the definition.
(I know that quick can be written as speedy)
'little boy plunged in scottish river' is the wordplay.
'little boy' becomes 'Ed' (nickname or 'little' form of Edward).
'plunged in' means one lot of letters goes inside another (I've seen this in other clues).
'scottish river' becomes 'spey' (I've seen this before).
'ed' going within 'spey' is 'SPEEDY'.
(Other definitions for speedy that I've seen before include "Expeditious" , "Moving quickly" , "Done quickly" , "in a hurry" , "Quick" .)