Conceals 150 trees (6)
I believe the answer is:
cloaks
'conceals' is the definition.
(I've seen this in another clue)
'150 trees' is the wordplay.
'150' becomes 'CL' (Roman numerals).
'trees' becomes 'oaks' (oak is a kind of tree).
'cl'+'oaks'='CLOAKS'
(Other definitions for cloaks that I've seen before include "Loose outer garments" , "Masks" , "Disguises" .)