Flipping rubbish hotel that mostly is solvent (7)
I believe the answer is:
naphtha
'is solvent' is the definition.
(I've seen this before)
'flipping rubbish hotel that mostly' is the wordplay.
'flipping' says the letters should be written backwards.
'rubbish' becomes 'pan' (both can mean to criticise heavily).
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'mostly' means to remove the last letter.
'pan' backwards is 'nap'.
'that' with its final letter removed is 'tha'.
'nap'+'h'+'tha'='NAPHTHA'
(Other definitions for naphtha that I've seen before include "Flammable liquid used as solvent" , "Inflammable distilled liquid" , "Inflammable oil" , "Bad-smelling distillate of carbon" , "Flammable liquid hydrocarbon mixture" .)