Son’s delighted — heading off, full of energy (5)
I believe the answer is:
sappy
'son's delighted heading off full of energy' is the definition.
I can't tell whether this definition defines the answer.
'son's delighted heading off' is the wordplay.
'delighted' becomes 'happy' (I've seen this before).
'heading off' means to remove the first letter.
'happy' with its first letter taken away is 'appy'.
's'+'appy'='SAPPY'
Can you help me to learn more?
(Other definitions for sappy that I've seen before include "Succulent" , "Full of juice" .)