Said ... "Colonel returning barrel!" (3)
I believe the answer is:
tun
'barrel' is the definition.
(I know that tun is a type of barrel)
'said colonel returning' is the wordplay.
'said' becomes 't' (I've seen this in other clues).
'colonel' becomes 'nu' (I am not sure about this - if you are sure you should believe this answer much more).
'returning' is a reversal indicator.
'nu' backwards is 'un'.
't'+'un'='TUN'
Can you help me to learn more?
(Other definitions for tun that I've seen before include "butt" , "Barrel holding many gallons" , "vessel" , "'Large cask, for beer perhaps (3)'" , "Container of beer" .)