Thin Frenchman with brilliant speed (8)
I believe the answer is:
macerate
'thin' is the definition.
(thesaurus)
'frenchman with brilliant speed' is the wordplay.
'frenchman' becomes 'm' (abbreviation for Monsieur - like 'Mr').
'with' is a charade indicator (letters next to each other).
'brilliant' becomes 'ace' (I've seen this before).
'speed' becomes 'rate' (the speed at which something happens).
'm'+'ace'+'rate'='MACERATE'
(Other definitions for macerate that I've seen before include "Steep in liquid like cream tea" , "Soak" , "Soften food by soaking" , "Cream tea (anag) -- soften by steeping in liquid" .)