One of five kids with constant energy gets fruit (6)
I believe the answer is:
quince
'fruit' is the definition.
(type of fruit)
'one of five kids with constant energy' is the wordplay.
'one of five kids' becomes 'quin' (short for quintuplet).
'with' means one lot of letters go next to another.
'constant' becomes 'c' (mathematical abbreviation).
'energy' becomes 'e' (physics symbol as in E=mc2).
'quin'+'c'+'e'='QUINCE'
'gets' acts as a link.
(Other definitions for quince that I've seen before include "tree" , "sliced fruit for newly-weds" , "part-time Athenian dramatist" , "Peter -- (A Midsummer Night's Dream)" , "Fruit for jelly" .)