Really happy with what Thomas the Tank Engine did? (7)
I believe the answer is:
chuffed
I believe this clue is a double definition.
'really happy' is the first definition.
(chuffed can mean very happy)
'what thomas the tank engine did?' is the second definition.
(steam engines chuff)
'with' acts as a link.
(Other definitions for chuffed that I've seen before include "to have gone like a train?" , "what bad wolf did?" , "Over the moon" , "Very glad" , "Very pleased (colloq.)" .)