Don't hand over, not having lost one's grip (8)
I believe the answer is:
'don't hand' is the definition.
The answer and definition can be both to do with ownership as well as being verbs in their base form.
Perhaps you can see an association between them that I can't see?
'over not having lost one's grip' is the wordplay.
'over not' becomes 'with' (I can't explain this - if you can you should give a lot more credence to this answer).
'having lost' suggests deleting specific letters.
'one' becomes 'a'.
'grip' becomes 'a-hold' (I've seen this in another clue).
'ahold' with 'a' removed is 'hold'.
Can you help me to learn more?
(Other definitions for withhold that I've seen before include "Keep in reserve, do not release", "Refrain from revealing", "Keep back", "Keep for oneself", "restrain".)