One cart adapted to carry trophies returned lacking balance (10)
I believe the answer is:
asymmetric
'lacking balance' is the definition.
(asymmetric can mean one-sided or imbalanced)
'one cart adapted to carry trophies returned' is the wordplay.
'one' becomes 'i' (Roman numeral).
'adapted' indicates an anagram.
'to carry' means one lot of letters goes inside another.
'trophies' becomes 'Emmys' (Emmy Awards).
'returned' says the letters should be written in reverse.
'emmys' written backwards gives 'symme'.
'i'+'cart'='icart'
'icart' with letters rearranged gives 'atric'.
'atric' going around 'symme' is 'ASYMMETRIC'.
(Other definitions for asymmetric that I've seen before include "kind of bars frequented by gymnasts" , "Not regularly shaped" , "crooked" , "women-only bars" , "Lopsided" .)