Old, imprisoned? One escapes, still able to achieve goal (6)
I believe the answer is:
onside
'escapes still able to achieve goal' is the definition.
I can't judge whether this definition defines the answer.
'old imprisoned? one' is the wordplay.
'old' becomes 'sid' (I can't explain this - if you can you should give a lot more credence to this answer).
'imprisoned?' indicates putting letters inside.
'sid' inserted within 'one' is 'ONSIDE'.
Can you help me to learn more?
(Other definitions for onside that I've seen before include "in a good position" , "in position to proceed?" , "in a position to score?" , "(In football) not in forbidden zone" , "Legally positioned on field" .)