Pipe a monster, not drained (7)
I believe the answer is:
hydrant
'pipe' is the definition.
(hydrant is a kind of pipe)
'a monster not drained' is the wordplay.
'a monster' becomes 'hydra' (Hydra is an example**).
'drained' says to hollow out the word (remove centre letters).
'not' with its centre taken out is 'nt'.
'hydra'+'nt'='HYDRANT'
(Other definitions for hydrant that I've seen before include "Firefighter's connection" , "Water source" , "Water tap for drawing from the mains" , "Water-pipe on the street for the fire brigade" , "Outlet from water main (for firefighters etc)" .)