Of organisms, one in horse parasite I primarily control (6)
I believe the answer is:
biotic
'of organisms one in horse parasite i primarily control' is the definition.
I can't tell whether this defines the answer.
'one in horse parasite i primarily control' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'horse parasite' becomes 'bot' (I can't justify this - if you can you should give a lot more credence to this answer).
'primarily' suggests taking the first letters.
The initial letter of 'control' is 'c'.
'i' going within 'bot' is 'biot'.
'biot'+'i'+'c'='BIOTIC'
Can you help me to learn more?
(Other definitions for biotic that I've seen before include "Pertaining to living organisms" , "Relating to living organisms" .)