It's not often the present combines with the past (3,3,4)
I believe the answer is:
now and then
'it's not often' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'present combines with the past' is the wordplay.
I cannot really understand how this works, but
'present' could be 'now' (both can mean the current period) and 'now' is present in the answer.
'with' could be 'and' and 'and' is located in the answer.
'past' could be 'then' (similar in meaning) and 'then' is found within the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'the' acts as a link.
Can you help me to learn more?
(Other definitions for now and then that I've seen before include "At times" , "Sometimes" , "making a historical contrast" , "Occasionally" .)