Assistant indicated horse up in second field belongs to composer (8)
I believe the answer is:
paganini
'to composer' is the definition.
(I've seen this before)
'assistant indicated horse up in second field' is the wordplay.
'assistant' becomes 'PA' (personal assistant).
'indicated horse up' becomes 'gan' (I can't justify this - if you can you should give a lot more credence to this answer).
'second field' becomes 'i' (and letter of 'field').
'pa'+'gan'+'in'+'i'='PAGANINI'
'belongs' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for paganini that I've seen before include "Niccolo --, Italian violinist" , "fellow on the fiddle" , "Famous Italian violinist" , "Violin virtuoso and composer" , "Italian barman" .)