Equivalent to apprentice initially feeding lightish brown horse (10)
I believe the answer is:
tantamount
'equivalent' is the definition.
(I've seen this before)
'to apprentice initially feeding lightish brown horse' is the wordplay.
'initially' suggests taking the first letters.
'feeding' is an insertion indicator.
'lightish brown' becomes 'tan' (I've seen this before).
'horse' becomes 'mount' (mount is a kind of horse**).
The first letters of 'to apprentice' is 'ta'.
'tan'+'mount'='tanmount'
'ta' placed into 'tanmount' is 'TANTAMOUNT'.
(Other definitions for tantamount that I've seen before include "Equivalent in seriousness (to)" , "Virtually equivalent (to)" , "Equivalent in value (to)" , "Equal in value" .)