Second footballer meeting monarch (6)
I believe the answer is:
backer
'second' is the definition.
(thesaurus)
'footballer meeting monarch' is the wordplay.
I cannot really understand how this works, but
'footballer' could be 'back' (back is a kind of footballer) and 'back' is located in the answer.
'monarch' could be 'er' (abbreviation for Elizabeth Regina, British Queen) and 'er' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for backer that I've seen before include "Person supporting another financially" , "'Supporter, usually provides finance (6)'" , "A supporter or a betting man" , "Financial supporter" , "Patron or supporter" .)