Reason for many a practice (5)
I believe the answer is:
cause
'reason' is the definition.
(I've seen this in another clue)
'many a practice' is the wordplay.
'many' becomes 'c' (C is the Roman numeral for 100).
'practice' becomes 'use' (use is a kind of practice).
'c'+'a'+'use'='CAUSE'
'for' is the link.
(Other definitions for cause that I've seen before include "It brings about an effect" , "Produce (an effect)" , "Reason; belief" , "Give rise to" , "why some people fight" .)