For, with this, it will be forever of benefit (4)
I believe the answer is:
good
'benefit' is the definition.
(I've seen this before)
'for with this it will be forever of' is the wordplay.
I cannot really see how this works, but
'be' could be 'go' (going is a kind of being) and 'go' is present in the answer.
'of' could be 'o' and 'o' is present in the answer.
A single letter 'd' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for good that I've seen before include "Of a high standard" , "Fine" , "Well done! Health-giving" , "Admirable" , "Beneficial" .)