Observe, in present and past, move up and down (6)
I believe the answer is:
seesaw
'move up and down' is the definition.
(synonyms)
'observe in present and past' is the wordplay.
I cannot really understand how this works, but
'observe' could be 'see' (I've seen this before) and 'see' is present in the answer.
The remaining letters 'saw' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for seesaw that I've seen before include "having to move from one extreme to the other" , "Children's up-and-down play structure" , "'Move up and down alternately'" , "Up-and-down children's game with plank on barrel" , "Up-and-down motion" .)