Gypsum plant behind an experimental facility (9)
I believe the answer is:
alabaster
'gypsum' is the definition.
(I've seen this before)
'plant behind an experimental facility' is the wordplay.
'plant' becomes 'aster' (aster is a kind of plant).
'behind' says to put letters next to each other.
'an' becomes 'a'.
'experimental facility' becomes 'lab' (I've seen this before).
'a'+'lab'='alab'
'aster' after 'alab' is 'ALABASTER'.
(Other definitions for alabaster that I've seen before include "Altar base (anagr.)" , "Sort of gypsum" , "White ornamental stone" , "White gypsum used for carving" , "that's smooth and white" .)