Liquid in the boiler (3)
I believe the answer is:
oil
'liquid' is the definition.
(I have seen 'Slippery liquid ' mean 'oil' so perhaps 'liquid' could also mean 'oil')
'in the boiler' is the wordplay.
'in' indicates the answer is hidden within the clue.
'OIL' can be found hidden inside 'boiler'.
(Other definitions for oil that I've seen before include "eg Olive" , "Grease" , "Slippery liquid" , "Anoint" , "Lubricate; fuel" .)