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 cannot tell whether it can be defined by this definition.
'child supports team' is the wordplay.
'child' becomes 'issue' (I've seen this before).
'supports' means one lot of letters go next to another (bottom letters provide a support for the top ones).
'team' becomes 'side' (both can mean a sports team).
'issue' put 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" .)