Exploit at length advantageously (8)
I believe the answer is:
usefully
'advantageously' is the definition.
(I've seen this before)
'exploit at length' is the wordplay.
'exploit' becomes 'use' (use can mean to exploit).
'at length' becomes 'fully' (I am not sure about this - if you are sure you should believe this answer much more).
'use'+'fully'='USEFULLY'
Can you help me to learn more?
(Other definitions for usefully that I've seen before include "quite profitably" , "in a way that's beneficial" , "in an advantageous way" , "in helpful manner" , "Advantageously" .)