Extremely tough skill, fencing with foil (6)
I believe the answer is:
thwart
'foil' is the definition.
(I know that foil can be written as thwart)
'extremely tough skill fencing with' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'skill' becomes 'art' (both can mean a talent).
'fencing' is an insertion indicator (to fence can mean to contain or put a limit round).
'with' becomes 'w' (short for with).
'tough' with its middle taken out is 'th'.
'th'+'art'='thart'
'thart' enclosing 'w' is 'THWART'.
(Other definitions for thwart that I've seen before include "Stymie" , "Prevent from succeeding" , "Oppose successfully" , "seat for oarsman" , "Snooker" .)