Look -- old boy's erected summerhouse (6)
I believe the answer is:
gazebo
'summerhouse' is the definition.
(I know that Gazebo is a summerhouse)
'look old boy's erected' is the wordplay.
'look' becomes 'gaze' (gazing is a kind of looking).
'old boy' becomes 'OB' (abbreviation for a private school 'old boy').
'erected' is a reversal indicator (in a down clue, letters go up).
'ob' in reverse letter order is 'bo'.
'gaze'+'bo'='GAZEBO'
(Other definitions for gazebo that I've seen before include "Metrical feet" , "Type of summerhouse" , "Small summerhouse or open hut" , "Garden shelter" , "Small summerhouse for shade" .)