In distribution of cards, I'm backing clubs often split (7)
I believe the answer is:
decimal
'often split' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I can't see?
'in distribution of cards i'm backing clubs' is the wordplay.
'in' indicates putting letters inside.
'distribution of cards' becomes 'deal' (cards dealt for a game).
'backing' means one lot of letters go next to another.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'im' after 'c' is 'cim'.
'deal' enclosing 'cim' is 'DECIMAL'.
(Other definitions for decimal that I've seen before include "Using powers of ten" , "Based on the number 10" , "Claimed, wrongly, to have this currency" , "System based on ten" , "A point in maths" .)