Over the moon here to have secret room (4,5)
I believe the answer is:
deep space
'over the moon here' is the definition.
'secret room' is the wordplay.
'secret' becomes 'deep'.
'room' becomes 'space' (I've seen this before**).
'deep'+'space'='DEEP SPACE'
'to have' acts as a link.
(Other definitions for deep space that I've seen before include "even devoid of 12 [THIN AIR]" , "Region beyond our atmosphere" , "void" .)