The pony with no energy troubled by snake (6)
I believe the answer is:
python
'snake' is the definition.
(python is a kind of snake)
'the pony with no energy troubled' is the wordplay.
'with no' suggests deleting specific letters.
'energy' becomes 'e' (physics symbol as in E=mc2).
'troubled' is an anagram indicator.
'the'+'pony'='thepony'
'thepony' with 'e' removed is 'thpony'.
'thpony' with letters rearranged gives 'PYTHON'.
'by' acts as a link.
(Other definitions for python that I've seen before include "Type of snake" , "Big snake that crushes its prey" , "Large snake that strangles its prey" , "Large constricting snake" , "Snake that kills its prey by squeezing" .)