Aggressive dog is able to back up promises (10)
I believe the answer is:
pugnacious
'aggressive' is the definition.
('pugnacious' can be similar in meaning to 'aggressive')
'dog is able to back up promises' is the wordplay.
'dog' becomes 'pug' (pug is a kind of dog).
'is able to' becomes 'can' (I've seen this before).
'back up' says the letters should be written backwards (I've seen 'backed up' mean this).
'promises' becomes 'ious' (IOU's, promises to pay a debt).
'can' in reverse letter order is 'nac'.
'pug'+'nac'+'ious'='PUGNACIOUS'
(Other definitions for pugnacious that I've seen before include "Aggressive" , "Quarrelsome" , "Combative" .)