Pie keeping hot with foil (6)
I believe the answer is:
thwart
'foil' is the definition.
(synonyms)
'pie keeping hot with' is the wordplay.
'pie' becomes 'tart' (I've seen this before).
'keeping' means one lot of letters goes inside another.
'hot' becomes 'h' (abbreviation eg on taps).
'with' becomes 'w' (abbreviation).
'h'+'w'='hw'
'tart' enclosing 'hw' is 'THWART'.
(Other definitions for thwart that I've seen before include "Frustrate and obstruct" , "Defeat" , "seat for oarsman" , "cross" , "Frustrate, prevent from doing" .)