Think over the extremes of loyalty on purpose (12)
I believe the answer is:
deliberately
'on purpose' is the definition.
(synonyms)
'think over the extremes of loyalty' is the wordplay.
'think over' becomes 'deliberate' (deliberating is a kind of thinking over).
'the extremes of' suggests removing the centre.
'loyalty' with its centre removed is 'ly'.
'deliberate'+'ly'='DELIBERATELY'
(Other definitions for deliberately that I've seen before include "Consciously and intentionally" , "as intended" , "On purpose" .)