President books the quarters within vast hotel compound (6)
I believe the answer is:
potash
'compound' is the definition.
(potash is a kind of compound)
'president books the quarters within vast hotel' is the wordplay.
'president' becomes 'p' (common abbreviation eg in VP).
'books' becomes 'OT' (abbreviation for Old Testament).
'the quarters within' says to take the centre (two quarters of the letters).
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
The middle letters of 'vast' are 'as'.
'p'+'ot'+'as'+'h'='POTASH'
(Other definitions for potash that I've seen before include "Caustic stuff" , "Agricultural chemical" , "Alkali used as fertiliser" , "feed in bed?" , "Potassium compound used as fertiliser" .)