Skilful concealing old British firm's shortfall (7)
I believe the answer is:
deficit
'shortfall' is the definition.
(I know that shortfall can be written as deficit)
'skilful concealing old british firm's' is the wordplay.
'skilful' becomes 'deft' (deft means skilful).
'concealing' means one lot of letters goes inside another.
'old british firm' becomes 'ICI' (Imperial Chemical Industries, former company).
'deft' enclosing 'ici' is 'DEFICIT'.
(Other definitions for deficit that I've seen before include "Arrears" , "Shortfall in revenue" , "There's something lacking" , "Financial shortfall" , "Amount of which total falls short" .)