A source of energy and relative growth (9)
I believe the answer is:
carbuncle
'growth' is the definition.
(I've seen this before)
'a source of energy and relative' is the wordplay.
'a source of energy' becomes 'carb' (short for carbohydrate).
'and' is a charade indicator (letters next to each other).
'relative' becomes 'uncle' (uncle is a kind of relative).
'carb'+'uncle'='CARBUNCLE'
(Other definitions for carbuncle that I've seen before include "Bright red gem; boil" , "Severe abscess in the skin" , "Architectural eyesore" , "Red gemstone; sort of boil" , "Skin eruption - gemstone" .)