When upset, brother gets poetic rhythm out of proportion (10)
I believe the answer is:
asymmetric
'out of proportion' is the definition.
(thesaurus)
'when upset brother gets poetic rhythm' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'upset' says the letters should be written backwards.
'brother' becomes 'my' (both are expressions of shock).
'gets' means one lot of letters go next to another.
'poetic rhythm' becomes 'metric' (I can't explain this - if you can you should give a lot more credence to this answer).
'my' back-to-front is 'ym'.
'as'+'ym'+'metric'='ASYMMETRIC'
Can you help me to learn more?
(Other definitions for asymmetric that I've seen before include "Unbalanced" , "women-only bars" , "Lopsided" , "crooked" , "Irregular" .)