Balance I found in contrary attitude (12)
I believe the answer is:
counterpoise
'balance' is the definition.
(I know that balance can be written as counterpoise)
'i found in contrary attitude' is the wordplay.
'found in' means one lot of letters goes inside another.
'contrary' becomes 'counter' (I've seen this before).
'attitude' becomes 'pose' (attitude can mean a pose or stance).
'counter'+'pose'='counterpose'
'i' placed within 'counterpose' is 'COUNTERPOISE'.
(Other definitions for counterpoise that I've seen before include "Offset" , "Weight balancing another" , "Balancing factor" , "balance" .)