String theory's first success with energy (5)
I believe the answer is:
twine
'string' is the definition.
('twine' can be a synonym of 'string')
'theory's first success with energy' is the wordplay.
'first' suggests taking the first letters.
'success' becomes 'win'.
'with' is a charade indicator (letters next to each other).
'energy' becomes 'e' (physics symbol as in E=mc2).
The first letter of 'theorys' is 't'.
't'+'win'+'e'='TWINE'
(Other definitions for twine that I've seen before include "Tough string" , "can get into knots" , "Knot" , "weaving" , "Coil" .)