Aware of prisoner's constant promises to pay (9)
I believe the answer is:
conscious
'aware of' is the definition.
('conscious' can be a synonym of 'aware of')
'prisoner's constant promises to pay' is the wordplay.
'prisoner's' becomes 'cons' (abbreviation for 'convict').
'constant' becomes 'c' (mathematical abbreviation).
'promises to pay' becomes 'ious' (more than one iou).
'cons'+'c'+'ious'='CONSCIOUS'
(Other definitions for conscious that I've seen before include "Having awareness, knowing" , "Having awareness of surroundings" , "Aware, of one's surroundings" , "Deliberate, intentional" , "Aware of surroundings, sensations and thoughts" .)