Happen to assume one's position (4,5)
I believe the answer is:
take place
'happen' is the definition.
(I know that happen can be written as take place)
'assume one's position' is the wordplay.
I cannot really see how this works, but
'assume' could be 'take' (synonyms) and 'take' is present in the answer.
'position' could be 'place' (place is a kind of position) and 'place' is present in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'to' is the link.
Can you help me to learn more?
(Other definitions for take place that I've seen before include "Talk peace (anag) - happen" , "Occur, happen - assume position" .)