Balance of one picture, so uneven (12)
I believe the answer is:
counterpoise
'balance' is the definition.
('counterpoise' can be a synonym of 'balance')
'one picture so uneven' is the wordplay.
'uneven' indicates anagramming the letters.
'one'+'picture'+'so'='onepictureso'
'onepictureso' anagrammed gives 'COUNTERPOISE'.
'of' is the link.
(Other definitions for counterpoise that I've seen before include "Balancing factor" , "Weight balancing another" , "balance" , "Offset" .)