Solvent the man mixed with oxygen last of all (8)
I believe the answer is:
methanol
'solvent' is the definition.
(methanol is used as a solvent)
'the man mixed with oxygen last of all' is the wordplay.
'mixed with' indicates anagramming the letters.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'last of' suggests the final letters.
The last letter of 'all' is 'l'.
'the'+'man'='theman'
'theman' with its letters anagrammed with 'o' gives 'methano'.
'methano'+'l'='METHANOL'
(Other definitions for methanol that I've seen before include "Toxic alcohol" , "Form of alcohol" , "Wood spirit" , "Dangerous product" .)