Water treatment centre a must-have for lengthy drought (5)
I believe the answer is:
hydro
'water treatment centre' is the definition.
(hydroelectric power plant)
'a must-have for lengthy drought' is the wordplay.
'a must-have for' says the answer is hidden in the clue (the answer is needed to spell part of the clue).
'HYDRO' can be found hidden inside 'lengthy drought'.
(Other definitions for hydro that I've seen before include "Hotel or clinic offering water treatment" , "Spa treatment 46 ac" , "hotel leads" , "Bath Spa?" , "Prefix indicating water" .)