Notices a fly going up and down (9)
I believe the answer is:
seesawing
'going up and down' is the definition.
(I've seen this before)
'notices a fly' is the wordplay.
'notices' becomes 'sees' (I've seen this in other clues).
'fly' becomes 'wing' (synonyms).
'sees'+'a'+'wing'='SEESAWING'
(Other definitions for seesawing that I've seen before include "may show fluctuation" , "Swinging" , "going up and down" .)