Fast start when Sheffield team includes Henry (3,9)
I believe the answer is:
ash wednesday
'fast start' is the definition.
(beginning of Lent)
'when sheffield team includes henry' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'sheffield team' becomes 'Wednesday' (Sheffield Wednesday FC).
'includes' means one lot of letters goes inside another.
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'as'+'wednesday'='aswednesday'
'aswednesday' placed around 'h' is 'ASH WEDNESDAY'.
(Other definitions for ash wednesday that I've seen before include "fast start" , "Date in church calendar" , "Start of Lent" , "Beginning of Lent" , "Poem by T.S. Eliot" .)