Complete change of mind concerning act (5,4)
I believe the answer is:
about turn
'concerning act' is the definition.
(I've seen this before)
'complete change of mind' is the wordplay.
I cannot quite see how this works, but
'change' could be 'turn' (turn is a kind of change) and 'turn' is located in the answer.
'of' could be 'o' and 'o' is found within the answer.
The remaining letters 'abut' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for about turn that I've seen before include "order to the troops" , "Face the opposite way" , "Major change of attitude" , "Pivot 180 degress" , "military command" .)