Substitutes concerned with soft strings on football boots? (8)
I believe the answer is:
replaces
'substitutes' is the definition.
('replace' can be a synonym of 'substitute')
'concerned with soft strings on football boots?' is the wordplay.
'concerned with' becomes 're' (re can mean regarding or about).
'soft' becomes 'p' (musical abbreviation for 'piano', soft).
'strings on football boots?' becomes 'laces' (I can't explain this - if you can you should give a lot more credence to this answer).
're'+'p'+'laces'='REPLACES'
Can you help me to learn more?
(Other definitions for replaces that I've seen before include "Substitutes one for another" , "Puts back" , "Substitutes, supplants" , "Substtitutes, supplants" .)