Stare, with old boy turning up in summerhouse (6)
I believe the answer is:
'summerhouse' is the definition.
(I know that summerhouse can be written as gazebo)
'stare with old boy turning up' is the wordplay.
'stare' becomes 'gaze' (synonyms).
'with' says to put letters next to each other.
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'turning up' says the letters should be written in reverse.
'ob' in reverse letter order is 'bo'.
'in' is the link.
(Other definitions for gazebo that I've seen before include "Summerhouse with views" , "Kind of garden summerhouse with wide view" , "Summer house with a wide view" , "Garden sumer-house with wide view" , "Belvedere" .)