Looking up, see heights (4)
I believe the answer is:
tops
'heights' is the definition.
('top' can be a synonym of 'height')
'looking up see' is the wordplay.
'looking up' says the letters should be written in reverse (in a down clue, letters go up).
'see' becomes 'spot' (to spot is to notice or catch sight of).
'spot' back-to-front is 'TOPS'.
(Other definitions for tops that I've seen before include "Betters" , "The best" , "Goes beyond" , "Max - spinning toys" , "Does better" .)