Was penthouse concealing a tree? (5)
I believe the answer is:
aspen
'a tree?' is the definition.
(aspen is a kind of tree)
'was penthouse concealing' is the wordplay.
'concealing' indicates the answer is hidden within the clue.
'ASPEN' is hidden within 'was penthouse'.
(Other definitions for aspen that I've seen before include "Colorado ski resort - tree" , "that's wooden" , "quill-like?" , "Tmorous" , "winter resort" .)