Thin material turned fat, horribly fat, with infusion of energy (7)
I believe the answer is:
taffeta
'thin material' is the definition.
(I know that taffeta is a type of thin material)
'turned fat horribly fat with infusion of energy' is the wordplay.
'turned' shows that the letters should be reversed in order.
'horribly' indicates an anagram (a horribly wrong version of the letters).
'with infusion of' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'fat' back-to-front is 'taf'.
'fat' with letters rearranged gives 'fta'.
'fta' going around 'e' is 'feta'.
'taf'+'feta'='TAFFETA'
(Other definitions for taffeta that I've seen before include "cloth" , "fine stuff indeed!" , "Stiff, silky fabric" , "Thin glossy silk fabric" , "Sheeny fabric" .)