One of the participants in the fight in the nursery (3,7)
I believe the answer is:
tin soldier
'one of the participants in' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'fight in the nursery' is the wordplay.
'fight' indicates an anagram (I've seen 'fighting' mean this).
'the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'nursery' becomes 'soldier' (I can't justify this - if you can you should give a lot more credence to this answer).
'in'+'t'='int'
'int' is an anagram of 'tin'.
'tin'+'soldier'='TIN SOLDIER'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for tin soldier that I've seen before include "[FAIRY STORY REF]" , "Toy fighting man, usually of lead (" , "Metal toy" .)