Look over the top of big old summerhouse (6)
I believe the answer is:
gazebo
'summerhouse' is the definition.
(I know that summerhouse can be written as gazebo)
'look over the top of big old' is the wordplay.
'look' becomes 'gaze' (gazing is a kind of looking).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'the top of' says to take the initial letters.
The first letters of 'big old' is 'bo'.
'gaze'+'bo'='GAZEBO'
(Other definitions for gazebo that I've seen before include "Summer house with a wide view" , "Garden viewpoint" , "Garden shelter" , "Raised summerhouse with a wide view" , "Kind of garden summerhouse with wide view" .)