Child supports team that hardly matters (4,5)
I believe the answer is:
side issue
'hardly matters' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'child supports team' is the wordplay.
'child' becomes 'issue' (I've seen this before).
'supports' is a charade indicator (letters next to each other) (bottom letters provide a support for the top ones).
'team' becomes 'side' (both can mean a sports team).
'issue' after 'side' is 'SIDE-ISSUE'.
'that' acts as a link.
Can you help me to learn more?
(Other definitions for side issue that I've seen before include "Illegitimate children" , "Incidental minor matter" , "That's not so important" , "Adjunct" , "matter of lesser importance" .)