Boy to eat gluttonously, gaining energy (6)
I believe the answer is:
george
'boy' is the definition.
'eat gluttonously gaining energy' is the wordplay.
'eat gluttonously' becomes 'gorge' (I've seen this before).
'gaining' is an insertion indicator.
'energy' becomes 'e' (physics symbol as in E=mc2).
'gorge' placed around 'e' is 'GEORGE'.
'to' acts as a link.
(Other definitions for george that I've seen before include "Best Actor Oscar winner 1970, for 'Patton'" , "The G in G. B. Shaw" , "Boy --; -- Clooney" , "one in 3 the 12 30 [DROP the DEAD DONKEY]" , "Patron saint of England" .)