Mother, terribly active, has a form of energy (13)
I believe the answer is:
thermodynamic
'a form of energy' is the definition.
I can't tell whether this definition defines the answer.
'mother terribly active' is the wordplay.
'terribly' indicates an anagram.
'active' becomes 'dynamic' (similar in meaning).
'mother' is an anagram of 'thermo'.
'thermo'+'dynamic'='THERMODYNAMIC'
'has' acts as a link.
Can you help me to learn more?
(Other definitions for thermodynamic that I've seen before include "Relating heat to energy" , "Temperature expressed on the Kelvin scale" .)