Microsoft program left behind former church (5)
I believe the answer is:
excel
'microsoft program' is the definition.
(Microsoft Excel)
'left behind former church' is the wordplay.
'left' becomes 'l' (common abbreviation).
'behind' says to put letters next to each other.
'former' becomes 'ex' (prefix meaning former or onetime).
'church' becomes 'CE' (abbreviation for Church of England).
'ex'+'ce'='exce'
'l' put after 'exce' is 'EXCEL'.
(Other definitions for excel that I've seen before include "Succeed, stand out" , "Surpass; shine" , "Be the best" , "Predominate in" , "Be very good at, better than others" .)